qa: rbd_mirror_fsx_compare.sh doesn't error out as expected

qa: rbd_mirror_fsx_compare.sh doesn't error out as expected

In mirror-thrash tests, one of the clusters can be effectively stopped
due to a watchdog bark while rbd_mirror_fsx_compare.sh is running and is
in the middle of the "wait for all images" loop:

2026-03-01T12:55:35.059 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:+ retrying_seconds=1040
2026-03-01T12:55:35.060 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:+ '[' 1040 -le 7200 ']'
2026-03-01T12:55:35.060 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:++ rbd --cluster cluster2 --pool mirror ls
2026-03-01T12:55:35.060 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:++ wc -l
2026-03-01T12:55:35.084 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:+ '[' 290 -ge 292 ']'
2026-03-01T12:55:35.084 INFO:tasks.workunit.cluster1.client.mirror.trial055.stderr:+ sleep 10
...
2026-03-01T12:55:49.568 INFO:tasks.daemonwatchdog.daemon_watchdog:thrasher.rbd_mirror.[cluster2] failed
2026-03-01T12:55:49.568 INFO:tasks.daemonwatchdog.daemon_watchdog:BARK! unmounting mounts and killing all daemons

In this scenario "rbd ls" is going to time out repeatedly, turning the
loop into up to a ~60-hour sleep (up to 720 iterations with a 5-minute
timeout + 10-second sleep per iteration).

Fixes: https://tracker.ceph.com/issues/75239
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>