Quarantine some tests (#11420)

Author: analogrelay

src/Servers/HttpSys/test/FunctionalTests/ServerTests.cs

@@ -148,6 +148,7 @@ public async Task Server_AppException_ClientReset()
         }
 
         [ConditionalFact]
+        [Flaky("https://github.com/aspnet/AspNetCore-Internal/issues/2415", FlakyOn.All)]
         public void Server_MultipleOutstandingSyncRequests_Success()
         {
             int requestLimit = 10;

src/Servers/Kestrel/test/FunctionalTests/Http2/ShutdownTests.cs

@@ -102,6 +102,7 @@ public async Task GracefulShutdownWaitsForRequestsToFinish()
         }
 
         [ConditionalFact]
+        [Flaky("https://github.com/aspnet/AspNetCore-Internal/issues/2667", FlakyOn.All)]
         public async Task GracefulTurnsAbortiveIfRequestsDoNotFinish()
         {
             var requestStarted = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);