[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).
I think it should be `recover { case e => promise.failure(e) }` 
Isnt it easier to have the `promise.success(events)` within the Try, and just do `recover { case e => promise.failure (e) }`