qa: Fix races with waiting for scrubs

qa: Fix races with waiting for scrubs

The trigger_scrub sets the last_scrub_stamp backwards to
force a scheduled scrub.  In a small window this stamp could get propagated
to the mgr.  A test failure occurred because wait_for_scrub() was confused
by seeing a backward moving date.

The most critical change is having wait_for_scrub() make sure that the
date advances past the previous in value.

A test failed because the random backoff kept delayed triggered scrub, so
set osd_scrub_backoff throughout.

Signed-off-by: David Zafman <dzafman@redhat.com>
(cherry picked from commit ae2c5331fb5ec3b2c81724d19259302903aabdf5)