Unstable test: KeysCollectable

In this appveyor build, the test failed with this output:

[xUnit.net 00:00:10.2106789]     Microsoft.VisualStudio.Threading.Tests.WeakKeyDictionaryTests.KeysCollectable [FAIL]
[xUnit.net 00:00:10.2108854]       Actual difference is -15865064.
[xUnit.net 00:00:10.2109402]       Expected: True
[xUnit.net 00:00:10.2109814]       Actual:   False
[xUnit.net 00:00:10.2119533]       Stack Trace:
[xUnit.net 00:00:10.2160650]         C:\projects\vs-threading\src\Microsoft.VisualStudio.Threading.Tests\WeakKeyDictionaryTests.cs(142,0): at Microsoft.VisualStudio.Threading.Tests.WeakKeyDictionaryTests.KeysCollectable()
[xUnit.net 00:00:10.2170002]       Output:
[xUnit.net 00:00:10.2170754]         Start 202275912, end 218140976, diff -15865064
Failed   Microsoft.VisualStudio.Threading.Tests.WeakKeyDictionaryTests.KeysCollectable
Error Message:
 Actual difference is -15865064.
Expected: True
Actual:   False
Stack Trace:
   at Microsoft.VisualStudio.Threading.Tests.WeakKeyDictionaryTests.KeysCollectable() in C:\projects\vs-threading\src\Microsoft.VisualStudio.Threading.Tests\WeakKeyDictionaryTests.cs:line 142
Standard Output Messages:
 Start 202275912, end 218140976, diff -15865064

Start with src/Microsoft.VisualStudio.Threading.Tests/WeakKeyDictionaryTests.cs at line 142 and run the KeysCollectable test. Compare the test output with the referenced AppVeyor failure, then identify what makes the result unstable. Done means the test no longer fails intermittently.