Fix race with TestApplicationErrorLogger (#26385)

Author: BrennanConroy

src/Servers/Kestrel/shared/test/TestApplicationErrorLogger.cs

@@ -106,8 +106,11 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
 
             if (_messageFilter?.Invoke(logMessage) == true)
             {
-                _messageFilterTcs.TrySetResult(logMessage);
+                var localTcs = _messageFilterTcs;
+                // need to set tcs to null before calling TrySetResult
+                // to prevent the next WaitForMessage possibly throwing for a non-null tcs
                 _messageFilterTcs = null;
+                localTcs.TrySetResult(logMessage);
             }
         }