Add unit tests to plugins for forward-compatibility

[jsync-swirlds]

Story Form

As a Block Node Engineer
I want to create thorough forward compatibility unit tests for each plugin
so that I have confidence that the block node will correctly handle messages from different versions of the block stream.

This story depends on #1883.

Technical Notes

• New Unit Tests (for each plugin; Publisher, Backfill, Verification, Files.recent, Files.Historical, Subscriber)
  • Verify that all of the "blanket blocks" pass through the plugin without change
    • BlockItems in == BlockItems out, in the same order
    • For each BlockItem, serialized bytes in == serialized bytes out
  • Verify that all of the "blanket blocks" can be parsed without error.
    • Verify both within the plugin, and the output of the plugin.
  • Verify that all block items in all "blanket blocks" are present in the output of the plugin.
    • This includes data sent to clients, batches in the messaging queue, notifications, and data stored on disk.