Tests: don't wait on publishes in PreserveOrder.Execute()

StackExchange.Redis.Tests/PreserveOrder.cs

@@ -41,7 +41,7 @@ public void Execute()
                     // it all goes to the server and back
                     for (int i = 0; i < COUNT; i++)
                     {
-                        sub.Publish(channel, i);
+                        sub.Publish(channel, i, CommandFlags.FireAndForget);
                     }
 
                     Log("Allowing time for delivery etc...");