[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).
Since this class would be called from Flume. Flume will create an instance of this class to run the sink - so theoretically it should not be private to this package.