[SPARK-3054][STREAMING] Add unit tests for Spark Sink.

This patch adds unit tests for Spark Sink.

It also removes the private[flume] for Spark Sink,
since the sink is instantiated from Flume configuration (looks like this is ignored by reflection which is used by
Flume, but we should still remove it anyway).
In that case, can you add a line right at the top that this class is not intended to be used inside Spark application. Just in case it appears in the scala docs / java docs. I will try to see how to eliminate this module from appearing in the docs.