Optimize the test by dropping the table early and by using only
one undo log thread, so that purge will be doing more useful work
and less busy work of suspending and resuming the worker threads.

The test used to cause shutdown timeout on 10.4 on buildbot, and
for me locally when using --mysqld=--innodb-sync-debug.
With these tweaks, it passes for me with --mysqld=--innodb-sync-debug.