qa/workunits/rbd: expand LevelSpec parsing coverage

qa/workunits/rbd: expand LevelSpec parsing coverage

Invoke "rbd mirror snapshot schedule ls -R" and "rbd mirror snapshot
schedule status" commands on all levels, consistently.  In particular,
make sure that an image level schedule is listed for a recursive query
at the pool level both before and after the schedule kicks in:

  $ rbd create --size 1G --mirror-image-mode snapshot -p foo bar
  $ rbd mirror snapshot schedule add -p foo --image bar 1m
  $ rbd mirror snapshot schedule ls -p foo -R
  POOL  NAMESPACE  IMAGE  SCHEDULE
  foo              bar    every 1m
  <wait for schedule to become visible in status>
  $ rbd mirror snapshot schedule ls -p foo -R
  POOL  NAMESPACE  IMAGE  SCHEDULE
  foo              bar    every 1m

Also, make sure that pool and image level status queries work:

  $ rbd mirror snapshot schedule status -p foo
  SCHEDULE TIME        IMAGE
  2022-03-04 07:14:00  foo/bar
  $ rbd mirror snapshot schedule status -p foo --image bar
  SCHEDULE TIME        IMAGE
  2022-03-04 07:14:00  foo/bar

Both of these issues are fixed by the previous commit.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>