Add test for serve-stale /w fetch-limits

Add test for serve-stale /w fetch-limits

Add a test case when fetch-limits are reached and we have stale data
in cache.

This test starts with a positive answer for 'data.example/TXT' in
cache.

1. Reload named.conf to set fetch limits.
2. Disable responses from the authoritative server.
3. Now send a batch of queries to the resolver, until hitting the
   fetch limits. We can detect this by looking at the response RCODE,
   at some point we will see SERVFAIL responses.
4. At that point we will turn on serve-stale.
5. Clients should see stale answers now.
6. An incoming query should not set the stale-refresh-time window,
   so a following query should still get a stale answer because of a
   resolver failure (and not because it was in the stale-refresh-time
   window).

(cherry picked from commit 11b74fc176f29820cbfff22e06ae9d8920b0a21c)