Minor changes to serve-stale tests

Minor changes to serve-stale tests

Minor changes are:
- Replace the "$RNDCCMD dumpdb" logic with "rndc_dumpdb" from
  conf.sh.common (it does the same thing).
- Update a comment to match the grep calls below it (comment said the
  rest should be expired, while the grep calls indicate that they
  are still in the cache, the comment now explains why).

(cherry picked from commit 86a1bbfe2893a76c72320dfd6c924a460468a5ee)