qa/suites/rbd: fix sporadic "rx-only direction" test failures
The existing
xmlstarlet sel -t -v '//mirror/peers/peer[1]/uuid')" = ""
test is bogus since a tx-only peer gets added after the remote
rbd-mirror daemon pings the local cluster. It happened to pass most
of the time because xmlstarlet filter just failed on an empty peers
array, producing the wrongly expected empty string by accident.
Fixes: https://tracker.ceph.com/issues/58688
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit
5cec2670be569ac9161743dec84f139b9e529d07)