qa: wait for purge queue operations to finish

qa: wait for purge queue operations to finish

TestFragmentation.test_deep_split relies on `num_strays`
to reach zero expecting that the purge threads would
have deleted the directory entries. However, checking
`num_strays` cannot be relied on since PurqeQueue merely
journals the purge item (see PurgeQueue::push) followed
by the StrayManager marking the stray as removed thereby
accounting `num_strays`.

So, add an additional condition to check if the purge
threads have finished processing items.

Fixes: http://tracker.ceph.com/issues/52487
Signed-off-by: Venky Shankar <vshankar@redhat.com>