seeing a lot of "MuxerIsCollected" failures on CI; it genuinely *is* collected...

seeing a lot of "MuxerIsCollected" failures on CI; it genuinely *is* collected in all local scenarios, so I'm suspicious that this is timing/load related; try to make it less twitchy

tests/StackExchange.Redis.Tests/GarbageCollectionTests.cs

@@ -39,6 +39,9 @@ public void MuxerIsCollected()
 //#endif
             var wr = new WeakReference(muxer);
             muxer = null;
+
+            ForceGC();
+            Thread.Sleep(2000); // GC is twitchy
             ForceGC();
 
             // should be collectable