MDEV-18224 MTR's internal check of the test case 'innodb.recovery_shutdown' failed due to extra #sql-ib*.ibd files

The test innodb.recovery_shutdown would occasionally fail,
because recovered incomplete transactions would be conflicting
with DROP TABLE, causing the background drop table queue to be invoked.

Add a slow shutdown before dropping the tables, so that the
recovered transactions will be rolled back. Starting with MDEV-14705,
normal shutdown would abort the rollback of recovered transactions.