Fixed concurrency issues in the TypeCache class.

[VladimirKhvostov]

TypeCache class has testAssemblyInfoCache and classInfoCache data members of type Dictionary<>.

Updates where under the lock, but reads were not, which sometimes causes the following error:

An exception occurred while invoking executor 'executor://mstestadapter/v2': One or more errors occurred. (Object reference not set to an instance of an object.) while running test
Stack:

---> System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Execution.TypeCache.GetClassInfo(TestMethod testMethod)
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Execution.TypeCache.GetTestMethodInfo(TestMethod testMethod, ITestContext testContext, Boolean captureDebugTraces)
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Execution.UnitTestRunner.RunSingleTest(TestMethod testMethod, IDictionary`2 testContextProperties)

This PR fixes this issue by switching to ConcurrentDictionary.

[Sanan07]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Haplois]

LGTM

[Haplois]

Merged. Thank you, @VladimirKhvostov, for your contribution!

[AbhitejJohn]

Thanks for fixing this all up @VladimirKhvostov.