在阅读了这个aspnetcore问题之后，发现问题出在我的 IHostedService 向Kafka提出请求的实现。
这个 StartAsync 方法正在执行任务，一直运行到请求完成。按照设计，这种方法意味着一发不可收拾，即启动任务，然后继续。更新了我的 KafkaAdmin 服务成为 BackgroundService ，覆盖 ExecuteAsync 方法，如下所示。随后，测试不再阻塞。

using System;
using System.Threading;
using System.Threading.Tasks;

using Confluent.Kafka;
using Confluent.Kafka.Admin;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

using KafkaAdmin.Kafka.Config;

namespace KafkaAdmin.Kafka
{
    public delegate IAdminClient KafkaAdminFactory(KafkaConfig config);

    /// <summary>Background Service to make a request from Kafka to create a topic</summary>
    public class KafkaAdminService : BackgroundService, IDisposable
    {
        private KafkaAdminFactory _Factory { get; set; }
        private ILogger<KafkaAdminService> _Logger { get; set; }
        private KafkaConfig _Config { get; set; }

        /// <summary>
        /// Retrieve KafkaConfig from appsettings
        /// </summary>
        /// <param name="config">Config POCO from appsettings file</param>
        /// <param name="clientFactory"><see cref="KafkaAdminFactory"/></param>
        /// <param name="logger">Logger instance</param>
        public KafkaAdminService(
            IOptions<KafkaConfig> config,
            KafkaAdminFactory clientFactory,
            ILogger<KafkaAdminService> logger)
        {
            if (clientFactory == null)
                throw new ArgumentNullException(nameof(clientFactory));

            if (config == null)
                throw new ArgumentNullException(nameof(config));

            _Config = config.Value ?? throw new ArgumentNullException(nameof(config));
            _Factory = clientFactory ?? throw new ArgumentNullException(nameof(clientFactory));
            _Logger = logger ?? throw new ArgumentNullException(nameof(logger));
        }

        /// <summary>
        /// Create a Kafka topic if it does not already exist
        /// </summary>
        /// <param name="token">Cancellation token required by IHostedService</param>
        /// <exception name="CreateTopicsException">
        /// Thrown for exceptions encountered except duplicate topic
        /// </exception>
        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            using (var client = _Factory(_Config))
            {
                try
                {
                    _Logger.LogInformation("Admin service trying to create Kafka Topic...");
                    _Logger.LogInformation($"Topic::{_Config.Topic.Name}, ReplicationCount::{_Config.Topic.ReplicationCount}, PartitionCount::{_Config.Topic.PartitionCount}");
                    _Logger.LogInformation($"Bootstrap Servers::{_Config.Consumer.BootstrapServers}");

                    await client.CreateTopicsAsync(new TopicSpecification[] {
                        new TopicSpecification {
                            Name = _Config.Topic.Name,
                            NumPartitions = _Config.Topic.PartitionCount,
                            ReplicationFactor = _Config.Topic.ReplicationCount
                        }
                    }, null);

                    _Logger.LogInformation($"Admin service successfully created topic {_Config.Topic.Name}");
                }
                catch (CreateTopicsException e)
                {
                    if (e.Results[0].Error.Code != ErrorCode.TopicAlreadyExists)
                    {
                        _Logger.LogInformation($"An error occured creating topic {_Config.Topic.Name}: {e.Results[0].Error.Reason}");
                        throw e;
                    }
                    else
                    {
                        _Logger.LogInformation($"Topic {_Config.Topic.Name} already exists");
                    }
                }
            }

            _Logger.LogInformation("Kafka Consumer thread started");

            await Task.CompletedTask;
        }

        /// <summary>
        /// Call base class dispose
        /// </summary>
        public override void Dispose()
        {
            base.Dispose();
        }
    }
}

还不明白为什么livewebapp会成功启动。为什么这只是testserver的一个问题？
我使用了confluent/cpKafkadocker图像。exmaple docker compose文件是：

---
version: "3.8"

services:
  zookeeper:
    image: confluentinc/cp-zookeeper:6.0.0
    hostname: zookeeper
    container_name: zookeeper
    ports:
      - "2181:2181"
    networks:
      - camnet
    environment:
      ZOOKEEPER_CLIENT_PORT: 2181
      ZOOKEEPER_TICK_TIME: 2000
      ZOOKEEPER_LOG4J_ROOT_LOGLEVEL: WARN

  kafka:
    image: confluentinc/cp-kafka:6.0.0
    hostname: kafka
    container_name: kafka
    depends_on:
      - zookeeper
    networks:
      - camnet
    environment:
      KAFKA_BROKER_ID: 1
      KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:9092
      KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT
      KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT
      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
      KAFKA_NUM_PARTITIONS: 3
      KAFKA_HEAP_OPTS: -Xmx512M -Xms512M
      KAFKA_LOG4J_ROOT_LOGLEVEL: WARN
      KAFKA_LOG4J_LOGGERS: "org.apache.zookeeper=WARN,org.apache.kafka=WARN,kafka=WARN,kafka.cluster=WARN,kafka.controller=WARN,kafka.coordinator=WARN,kafka.log=WARN,kafka.server=WARN,kafka.zookeeper=WARN,state.change.logger=WARN"

  netclient-test:
    build:
      context: ../
      dockerfile: docker/Dockerfile
    container_name: netclient-test
    image: dcs3spp/netclient
    networks:
      - camnet
    depends_on:
      - kafka
      - netclient-run
    entrypoint: []
    command:
      - bash
      - -c
      - |-
        echo 'Giving Kafka a bit of time to start up…'
        while ! nc -z kafka 9092;
        do
          sleep 1;
        done;

        echo 'Giving netclient-run a bit of time to start up…'
        while ! nc -z netclient-run 80;
        do
          sleep 1;
        done;

        echo .NET Client test container ready. Running test that uses WebApplicationFactory TestServer to start WebApp with KafkaAdmin background service
        echo This runs successfully in a local development environment on MacOS and Ubuntu Linux 16.04.
        echo This fails when running on a GitLab CI Server. It can be seen that the test server bootstraps the WebApp.....
        echo The KafkaAdmin background service blocks when requesting topic creation from the kafka service
        dotnet test --runtime linux-musl-x64 -c Release --no-restore  --nologo tests/KafkaAdmin.Kafka.IntegrationTests/

  netclient-run:
    build:
      context: ../
      dockerfile: docker/Dockerfile
    container_name: netclient-run
    image: dcs3spp/netclient
    networks:
      - camnet
    depends_on:
      - kafka
    entrypoint: []
    command:
      - bash
      - -c
      - |-
        echo 'Giving Kafka a bit of time to start up…'
        while ! nc -z kafka 9092;
        do
          sleep 1;
        done;
        echo .NET Run Web App Ready. Starting WebApp that contains KafkaAdmin background service.
        dotnet run --runtime linux-musl-x64 -c Release --no-restore --project src/KafkaAdmin.WebApp/

networks:
  camnet:

我还创建了一个存储库来演示问题和修复。主分支包含显示问题的源代码。feat/fix分支包含包含修复的源代码。
希望这能帮助其他遇到类似问题的人！