Add system tests for stale-answer-client-timeout

Add system tests for stale-answer-client-timeout

This commit add 4 tests for the new option:
	1. Test default configuration of stale-answer-client-timeout, a
	   value of 1.8 seconds, with stale-refresh-time disabled.

	2. Test disabling of stale-answer-client-timeout.

	3. Test stale-answer-client-timeout with a value of zero, in this
	   case we take advantage of a log entry which shows that a stale
	   answer was promptly used before an attempt to refresh the RRset
	   is made. We also check, by activating a disabled authoritative
	   server, that the RRset was successfully refreshed after that.

	4. Test stale-answer-client-timeout 0 with stale-refresh-time 4, in
	   this test we want to ensure a couple things:

	   - If we have a stale RRSet entry in cache, a request must be
		 promptly answered with this data, while BIND must also attempt
		 to refresh the RRSet in background.

	   - If the attempt to refresh the RRSet times out, the RRSet must
		 have its stale-refresh-time window activated.

	   - If a new request for the same RRSet arrives, it must be
		 promptly answered with stale data due to stale-refresh-time
		 being active for this RRSet, in this case no attempt to refresh
		 the RRSet is made.

	   - Enable authoritative server, ensure that the RRSet was not
		 refreshed, to honor stale-refresh-time.

	   - Wait for stale-refresh-window time pass, send another request
		 for the same RRSet, this time we expect the answer to be the
		 stale entry in cache being hit due to
		 stale-answer-client-timeout 0.

	    - Send another request, this time we expect the answer to be an
		  active RRSet, since it must have been refreshed during the
		  previous request.

(cherry picked from commit 35fd039d038419d0301737f3a9afdd2310a81234)