Adjusted ancient rrset system test

Adjusted ancient rrset system test

Before the stale-refresh-time feature, the system test for ancient rrset
was somewhat based on the average time the previous tests and queries
were taking, thus not very precise.

After the addition of stale-refresh-time the system test for ancient
rrset started to fail since the queries for stale records (low
max-stale-ttl) were not taking the time to do a full resolution
anymore, since the answers now were coming from the cache (because the
rrset were stale and within stale-refresh-time window after the
previous resolution failure).

To handle this, the correct time to wait before rrset become ancient is
calculated from max-stale-ttl configuration plus the TTL set in the
rrset used in the tests (ans2/ans.pl).

Then before sending queries for ancient rrset, we check if we need to
sleep enough to ensure those rrset will be marked as ancient.