test/rbd-mirror: pool watcher registration error might result in race

test/rbd-mirror: pool watcher registration error might result in race

The init finish context should be swapped out before it attempts to
re-register the watcher. This affects the test case which mocks the
timer to fire immediately instead of after 30 seconds.

Fixes: https://tracker.ceph.com/issues/46669
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit c89d31ebf6c412d609123979c63ebc600b70e179)

Conflicts:
	src/tools/rbd_mirror/PoolWatcher.cc
- nautilus uses Mutex::Locker where master has std::lock_guard