qa/tasks/thrashosds-health: whitelist PG_BACKFILL_FULL

qa/tasks/thrashosds-health: whitelist PG_BACKFILL_FULL

rados/thrash-old-clients tests are failing due to PG_BACKFILL
error.

The low space error hindering backfill error is a expected
transitory state which resolves by itself when the PGs
are migrated out of the OSD during the test runs, freeing
up the needed space.

Yet, teuthology seems to pick up these PG_BACKFILL errors
and error out the test.

The solution is to add these expected errors into the
ignore list

Fixes: https://tracker.ceph.com/issues/65450
Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>