HA+MT unit test,MtHttpClientTest.workPauseResumeShutdown, fails sporadically

The test fails with the following output:

client_mt_unittests.cc:731: Failure
Expected: (clientRR->thread_id_) != (main_thread_id), actual: "139805142284160" vs "139805142284160"
[  FAILED  ] MtHttpClientTest.workPauseResumeShutdown (43 ms)

This failure occurs because the assertion that the thread-id in the response must not be the main thread-id is not true for this test. This test exercises pausing and resuming HA+MT client threads. When the main thread pauses, it calls IOService::poll() to execute any ready events which means that periodically it will the thread-id which builds the response. The assertion should removed or modified to check that most requests are handled by worker threads.