Unstable test: UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders

This Azure Pipelines build experienced a failure in the UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders test.

2018-09-22T06:48:12.4851424Z Failed   UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders
2018-09-22T06:48:12.4851652Z Error Message:
2018-09-22T06:48:12.4851860Z  System.AggregateException : One or more errors occurred.
2018-09-22T06:48:12.4852066Z ---- Assert.True() Failure
2018-09-22T06:48:12.4852838Z Expected: True
2018-09-22T06:48:12.4853670Z Actual:   False
2018-09-22T06:48:12.4853998Z Stack Trace:
2018-09-22T06:48:12.4854266Z    at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
2018-09-22T06:48:12.4854618Z    at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
2018-09-22T06:48:12.4855074Z    at Microsoft.VisualStudio.Threading.Tests.AsyncReaderWriterLockTests.UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders() in D:\a\1\s\src\Microsoft.VisualStudio.Threading.Tests\AsyncReaderWriterLockTests.cs:line 1482
2018-09-22T06:48:12.4855462Z ----- Inner Stack Trace -----
2018-09-22T06:48:12.4856105Z    at Microsoft.VisualStudio.Threading.Tests.AsyncReaderWriterLockTests.<>c__DisplayClass74_0.<<UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders>b__0>d.MoveNext() in D:\a\1\s\src\Microsoft.VisualStudio.Threading.Tests\AsyncReaderWriterLockTests.cs:line 1468

Start with src/Microsoft.VisualStudio.Threading.Tests/AsyncReaderWriterLockTests.cs, especially UpgradeableReadLockAsyncSynchronousReleaseAllowsOtherUpgradeableReaders around lines 1468 and 1482. Run this test repeatedly and inspect the reported Assert.True failure and Azure Pipelines log. Done means the test no longer fails intermittently while still checking synchronous release behavior.