add FlakyAttribute to mark flaky tests (#1222)

part of aspnet/AspNetCore#8237

src/Logging/Logging.Testing/src/LoggedTest/LoggedTestBase.cs

@@ -91,6 +91,14 @@ namespace Microsoft.Extensions.Logging.Testing
 
         public virtual void Dispose()
         {
+            if(_testLog == null)
+            {
+                // It seems like sometimes the MSBuild goop that adds the test framework can end up in a bad state and not actually add it
+                // Not sure yet why that happens but the exception isn't clear so I'm adding this error so we can detect it better.
+                // -anurse
+                throw new InvalidOperationException("LoggedTest base class was used but nothing initialized it! The test framework may not be enabled. Try cleaning your 'obj' directory.");
+            }
+
             _initializationException?.Throw();
             _testLog.Dispose();
         }