我认为你应该测试一下这个方法 :create_wallet 将被称为 only one time 在用户创建之后。然后，您肯定可以为该方法创建另一个测试用例 :create_wallet 它本身

require "test_helper"
require "active_support/testing/method_call_assertions"

class UserTest < ActiveSupport::TestCase
  include ActiveSupport::Testing::MethodCallAssertions

  test "after create callback" do
    user = User.new(...)
    assert_called(user, :create_wallet, times: 1) do
      user.save
    end
  end

  test "create wallet" do
    user = User.new(...)
    assert_changes -> {user.wallet.blank?}, from: true, to: false do
      user.save
    end

    # or assert_equal
    user = User.create(...) 
    assert_equal Wallet.last.user_id, user.id
    assert_equal 1, Wallet.where(user_id: user.id).count
  end
end