runtime: Failure in System.Diagnostics.Tests.ActivityTests.DiagnosticSourceStartStop

@joperezr can you find the right owner for DiagnosticSource? This gave us a red badge this morning. This test may need some logging adding, and leave enabled.

https://ci.dot.net/job/dotnet_corefx/job/master/job/outerloop_win7_release/lastCompletedBuild/consoleText

  xUnit.net console test runner (64-bit .NET Core)
  Copyright (C) 2014 Outercurve Foundation.
  
  Discovering: System.Diagnostics.DiagnosticSource.Tests
  Discovered:  System.Diagnostics.DiagnosticSource.Tests
  Starting:    System.Diagnostics.DiagnosticSource.Tests
     System.Diagnostics.Tests.ActivityTests.DiagnosticSourceStartStop [FAIL]
        Assert.True() Failure
        Expected: True
        Actual:   False
        Stack Trace:
           D:\j\workspace\outerloop_win---75d58f69\src\System.Diagnostics.DiagnosticSource\tests\ActivityTests.cs(378,0): at System.Diagnostics.Tests.ActivityTests.DiagnosticSourceStartStop()
  Finished:    System.Diagnostics.DiagnosticSource.Tests