[FLINK-26121][runtime] Adds clearUnhandledEvents() method

We experienced some ZK connection issue which resulted in a
SUSPEND and a RECONNECT event which were not handled and,
therefore, stayed in the queue blocking a thread for the
second event (because the capacity of the internally used
ArrayBlockingQueue is set to 1). Shutting down the driver
initiates a shutdown of the ExecutorService which is used
for the listeners internally (see
ZooKeeperLeaderRetrievalDriver.close()). The blocking thread
will cause a fatal error through an InterruptedException.
The test will fail due to the fatal error in the end.

Cleaning unhandled events at the end of the test run fix the issue. Additionally, the tests were aligned by moving generic
code into a helper method.

Author: XComp