Sometimes, the test would fail with a result difference for
the READ UNCOMMITTED read, because the incremental backup
would finish before redo log was written for all the rows
that were inserted in the second batch.

To fix that, cause a redo log write by creating another
transaction. The transaction rollback (which internally does commit)
will be flushed to the redo log, and before that, all the preceding
changes will be flushed to the redo log as well.