Add a test case to the dnssec system test to check that:
- a zone with a prepublished key is only signed with the active key.
- a zone with an inactive key but valid signatures retains those
signatures and does not add signatures from successor key.
- signatures are swapped in a zone when signatures of predecessor
inactive key are within the refresh interval.
(cherry picked from commit 35efbc270f)
e43d9f08b9