Add serve-stale test case for CNAME to A

Add serve-stale test case for CNAME to A

Add a serve-stale system test case where the authority changes a
CNAME RRset to A (at cname2.stale.test). The CNAME that is in the
cache is stale and should be refreshed. The target A record (at
a2.stale.test) has a longer TTL and is also still in the cache. The
next query should return the refreshed A RRset to the client.

Then the authority changes back the A RRset to CNAME. The A RRset
has become stale and should be refreshed. The next query should
return the refreshed CNAME RRset plus the already cached
a2.stale.test A record.

This test requires ns1 to allow dynamic updates to stale.test, and
prefetch to be disabled. The latter is to ensure the record is not
prefetched, but only refreshed when stale (and logs the expected
"an attempt to refresh the RRset" messages).