test/rgw: test_datalog_autotrim filters out new entries

test/rgw: test_datalog_autotrim filters out new entries

if other sync activity is racing with test_datalog_autotrim, it can
create new datalog entries after the 'datalog autotrim' command runs

instead of asserting that the datalog is empty after trim, assert that
any entries have a marker larger than the max-marker reported by
'datalog status' before the trim

Fixes: https://tracker.ceph.com/issues/45626
Signed-off-by: Casey Bodley <cbodley@redhat.com>