vinyl: test vylog error handling

Check that:
 - Error writing to vylog does not break vinyl permanently.
 - An index drop/truncate/create record that was not written to vylog
   due to a write error is buffered and flushed along with the next
   write.
 - A buffered index drop/truncate/create record that hasn't been flushed
   before restart is replayed on WAL recovery.

Closes #2490