Starting with MDEV-12288 in MariaDB Server 10.3,
the transaction identifiers on records will be reset on purge.
Because purge might or might not run to completion before shutdown,
it could happen that the bogus transaction identifier that our
test is writing will be reset by purge after restart, and the
expected warning message on SELECT will fail to appear.

We resolve the race condition by ensuring that purge runs to
completion before the shutdown.