Flaky CI tests

[Tratcher]

Now that WebListener is public and actually running on the CI and AppVeyor, some of the tests are flaky in those environments. Here's a running list of tests to investigate:

• ResponseBodyTests.ResponseBody_SecondWriteAsyncWithCanceledCancellationToken_CancelsAndAborts
• ResponseBodyTests.ResponseBodyWriteExceptions_ClientDisconnectsBeforeSecondWriteAsync_WriteThrows
• Microsoft.AspNetCore.Server.WebListener.ServerTests.Server_MultipleOutstandingSyncRequests_Success
• Microsoft.AspNetCore.Server.WebListener.FunctionalTests.ResponseCachingTests.Caching_MaxAge_Cached

[JunTaoLuo]

• ResponseBodyTests.ResponseBody_WriteContentLengthTooMuchWritten_Throws

[Tratcher]

These and a few others got worse when we merged the two test projects and more tests ran in parallel. Adding [assembly: CollectionBehavior(DisableTestParallelization = true)] helped significantly, but we should work out the conflicts eventually.

[JunTaoLuo]

• ResponseBodyTests.ResponseBody_WriteContentLengthNotEnoughWritten_Aborts

[jbagga]

• ResponseBodyTests.ResponseBodyWriteExceptions_ClientDisconnectsBeforeSecondWrite_WriteThrows
  Assert.Throws() Failure
  Expected: typeof(System.IO.IOException)
  Actual: (No exception was thrown)

[jbagga]

MessagePumpTests.UseDefaultAddress_WhenNoServerAddressAndNoDirectConfiguration
Failure details: http://aspnetci/viewLog.html?buildId=192016&tab=buildResultsDiv&buildTypeId=Lite_UniverseTest

@JunTaoLuo could you investigate this?