//creating mockContent for httpRequest
MockHttpContent mockHttpContent = new MockHttpContent();
String content = new String("requestBody");
mockHttpContent.setContent(str.getBytes());

//mocking httpResponse and linking to httpRequest's execution
HttpTransport transport =
        new MockHttpTransport() {
          @Override
          public LowLevelHttpRequest buildRequest(String method, String url) throws IOException {
            return new MockLowLevelHttpRequest() {
              @Override
              public LowLevelHttpResponse execute() throws IOException {
                MockLowLevelHttpResponse result = new MockLowLevelHttpResponse();
                result.setContent("responseBody");
                result.setContentEncoding("UTF-8");//this is very important
                result.setHeaderNames(List.of("header1","header2"));
                result.setHeaderValues(List.of("header1","header2"));
                return result;
              }
            };
          }
        };
HttpRequest httpRequest = transport.createRequestFactory().buildPostRequest(HttpTesting.SIMPLE_GENERIC_URL,mockHttpContent);

//getting httpResponse from httpRequest
 httpResponse = httpRequest.execute();

//condition to verify the content (body) of the response
assertEquals("responseBody",IOUtils.toString(httpResponse.getContent()));