tests: fix race condition testing auto scrub

tests: fix race condition testing auto scrub

When testing auto scrub, waiting 20 seconds for the scrub to complete is
sometimes not enough and creates false negatives.

Split wait_for_scrub out of the repair helper so that it can be used to
wait for the scrub to happen instead of using a timer.

The scrub timestamp is obtained after removing the object, therefore
there is a chance for the scrub to be finished already. But since auto
scrub is scheduled every 5 seconds, it will only make the test wait an
extra 5 seconds and not hang forever.

http://tracker.ceph.com/issues/13592

Signed-off-by: Xinze Chi <xinze@xsky.com>
Signed-off-by: Loic Dachary <loic@dachary.org>