[SPARK-3912][Streaming] Fixed flakyFlumeStreamSuite

@harishreedharan @pwendell
See JIRA for diagnosis of the problem
https://issues.apache.org/jira/browse/SPARK-3912

The solution was to reimplement it.
1. Find a free port (by binding and releasing a server-scoket), and then use that port
2. Remove thread.sleep()s, instead repeatedly try to create a sender and send data and check whether data was sent. Use eventually() to minimize waiting time.
3. Check whether all the data was received, without caring about batches.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #2773 from tdas/flume-test-fix and squashes the following commits:

93cd7f6 [Tathagata Das] Reimplimented FlumeStreamSuite to be more robust.