Recently, PySpark ML streaming tests have been flaky, most likely because of the batches not being processed in time.  Proposal: Replace the use of _ssc_wait (which waits for a fixed amount of time) with a method which waits for a fixed amount of time but can terminate early based on a termination condition method.  With this, we can extend the waiting period (to make tests less flaky) but also stop early when possible (making tests faster on average, which I verified locally).

CC: mengxr tdas freeman-lab

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #8087 from jkbradley/streaming-ml-tests.