Fix Test Failures from MockNioTransport Logger (elastic#42545)

* This call can fail when it tries to re-schedule the timeout check
after the threadpool was shut down already failing tests with
RejectedExecutionException

Author: original-brownbear

test/framework/src/main/java/org/elasticsearch/transport/nio/MockNioTransport.java

@@ -372,7 +372,7 @@ private void logLongRunningExecutions() {
                 }
             }
             if (stopped == false) {
-                threadPool.schedule(this::logLongRunningExecutions, CHECK_INTERVAL, ThreadPool.Names.GENERIC);
+                threadPool.scheduleUnlessShuttingDown(CHECK_INTERVAL, ThreadPool.Names.GENERIC, this::logLongRunningExecutions);
             }
         }