Added system test for stale-refresh-time

Added system test for stale-refresh-time

This test works as follow:
- Query for data.example rrset.
- Sleep until its TTL expires (2 secs).
- Disable authoritative server.
- Query for data.example again.
- Since server is down, answer come from stale cache, which has
  a configured stale-answer-ttl of 3 seconds.
- Enable authoritative server.
- Query for data.example again
- Since last query before activating authoritative server failed, and
  since 'stale-refresh-time' seconds hasn't elapsed yet, answer should
  come from stale cache and not from the authoritative server.