System.Net.WebSockets.Client.Tests hangs on single core Linux

[wfurt]

https://mc.dot.net/#/product/netcore/30/source/official~2Fcorefx~2Fmaster~2F/type/test~2Ffunctional~2Fcli~2F/build/20181002.03

This is causing consistent catastrophic failures on RedHat.69.
Because of unrelated issues, RH6 is only OS running on single core VM.
(daily failure since August 15)

CI runs fail only on Redhat.6 but after some investigation I was able to reproduce it on Ubuntu as well after changing my VM to have only one core. (same test (and exactly same assembly) was passing with multiple cores).

furt@Ubuntu:~/git/wfurt-corefx2/src/System.Net.WebSockets.Client/tests$ ../../../Tools/msbuild.sh /t:rebuildandtest /p:outerloop=true
 ~/git/wfurt-corefx2/bin/tests/System.Net.WebSockets.Client.Tests/netcoreapp-Linux-Debug-x64 ~/git/wfurt-corefx2/src/System.Net.WebSockets.Client/tests
  xUnit.net Console Runner v2.4.1-pre.build.4059 (64-bit .NET Core 4.6.26928.03)
    Discovering: System.Net.WebSockets.Client.Tests (method display = ClassAndMethod, method display options = None)
    Discovered:  System.Net.WebSockets.Client.Tests (found 76 of 79 test cases)
    Starting:    System.Net.WebSockets.Client.Tests (parallel test collections = on, max threads = 1)
     System.Net.WebSockets.Client.Tests: [Long Running Test] 'System.Net.WebSockets.Client.Tests.CloseTest.CloseOutputAsync_ClientInitiated_CanReceive_CanClose', Elapsed: 00:02:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.MemorySendReceiveTest.ReceiveAsync_MultipleOutstandingReceiveOperations_Throws', Elapsed: 00:02:08
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.AbortTest.Abort_CloseAndAbort_Success', Elapsed: 00:02:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.ConnectTest.ConnectAsync_NotWebSocketServer_ThrowsWebSocketExceptionWithMessage', Elapsed: 00:02:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.ArraySegmentSendReceiveTest.SendReceive_VaryingLengthBuffers_Success', Elapsed: 00:02:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.ClientWebSocketOptionsTests.Proxy_SetNull_ConnectsSuccessfully', Elapsed: 00:02:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.CancelTest.ConnectAsync_Cancel_ThrowsWebSocketExceptionWithMessage', Elapsed: 00:02:08
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.KeepAliveTest.KeepAlive_LongDelayBetweenSendReceives_Succeeds', Elapsed: 00:02:11

...

  [Long Running Test] 'System.Net.WebSockets.Client.Tests.ClientWebSocketOptionsTests.Proxy_SetNull_ConnectsSuccessfully', Elapsed: 00:12:10
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.CancelTest.ConnectAsync_Cancel_ThrowsWebSocketExceptionWithMessage', Elapsed: 00:12:08
  [Long Running Test] 'System.Net.WebSockets.Client.Tests.KeepAliveTest.KeepAlive_LongDelayBetweenSendReceives_Succeeds', Elapsed: 00:12:11
....
``