Test failures: Microsoft.Extensions.Caching.Memory flakiness

[maryamariyan]

Microsoft.Extensions.Caching.Memory.MemoryCacheSetAndRemoveTests.GetAndSet_AreThreadSafe_AndUpdatesNeverLeavesNullValues [FAIL]
      Assert.False() Failure
      Expected: False
      Actual:   True
      Stack Trace:
        /_/src/libraries/Microsoft.Extensions.Caching.Memory/tests/MemoryCacheSetAndRemoveTests.cs(488,0): at Microsoft.Extensions.Caching.Memory.MemoryCacheSetAndRemoveTests.GetAndSet_AreThreadSafe_AndUpdatesNeverLeavesNullValues()
        /_/src/mono/netcore/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs(339,0): at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

https://helix.dot.net/api/2019-06-17/jobs/e7ed99d0-5a25-4792-8b58-04c5a0009c69/workitems/Microsoft.Extensions.Caching.Memory.Tests/console

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UPDATE:
Failures not just on Mono. The flakiness reported in #34128 and disabled in PR #34142

[ericstj]

Duping this to #34128 since it has more data.

[ericstj]

Reopen since tests are disabled against this

[ericstj]

There may be more tests failing than just these. I sampled some of the logs listed in #34128 and see a couple other tests failing:

https://helix.dot.net/api/2019-06-17/jobs/69862130-a475-4556-96d7-896801bfcfec/workitems/Microsoft.Extensions.Caching.Memory.Tests/files/console.ed98370f.log

Microsoft.Extensions.Caching.Memory.CapacityTests.CompactsToLessThanLowWatermarkUsingLRUWhenHighWatermarkExceeded [FAIL]
      Assert.True() Failure
      Expected: True
      Actual:   False
      Stack Trace:
        /_/src/libraries/Microsoft.Extensions.Caching.Memory/tests/CapacityTests.cs(378,0): at Microsoft.Extensions.Caching.Memory.CapacityTests.CompactsToLessThanLowWatermarkUsingLRUWhenHighWatermarkExceeded()

https://helix.dot.net/api/2019-06-17/jobs/7df59f03-64bf-479e-a50e-680591bfadde/workitems/Microsoft.Extensions.Caching.Memory.Tests/files/console.b7698877.log

    Microsoft.Extensions.Caching.Memory.CapacityTests.ExpiringEntryDecreasesCacheSize [FAIL]
      Assert.True() Failure
      Expected: True
      Actual:   False
      Stack Trace:
        /_/src/libraries/Microsoft.Extensions.Caching.Memory/tests/CapacityTests.cs(338,0): at Microsoft.Extensions.Caching.Memory.CapacityTests.ExpiringEntryDecreasesCacheSize()

I wonder if there is something broken with how we're testing these in dotnet/runtime? Maybe concurrency differences and shared state?