qa/standalone: fixing the timings when waiting for deep-scrub to start

qa/standalone: fixing the timings when waiting for deep-scrub to start

initiate_and_fetch_state() initiates a scrub, then polls the published
PG state looking for 'scrubbing'. Calling flush_pg_stats() as part of
the polling process might cause the scrub and the following recovery to
be missed altogether.

Note: this polling mechanism is definitely not robust. Will be
redesigned in the future.

Fixes: https://tracker.ceph.com/issues/51581
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>
(cherry picked from commit ed45acee34435611f8dea7f77fde54a6586cf6d9)