srv_purge_should_exit(): Remove the parameter n_purged.
If we happened to have n_purged==0 while some transaction was still
active, and then that transaction was added to the history list,
we were prematurely stopping the purge. It is more appropriate to
first check for trx_sys.any_active_transactions() == 0
(this count can only decrease during shutdown) and then for
trx_sys.history_size() == 0 (that count typically decreases, but
can increase when any remaining active transactions are committed
or rolled back).

innodb.dml_purge: Remove a server restart, and explicitly wait for
purge, and use FLUSH TABLE FOR EXPORT to read the file contents.
This will make the test run faster, easier to debug, and also
allow it to run with --embedded. This might also help repeat
MDEV-11802 better. The issue MDEV-13603 remains will remain tested
by innodb.table_flags.