Add serve-stale test case for GL #3950

Add serve-stale test case for GL #3950

Add a test case where when priming the cache with a slow authoritative
resolver, the stale-answer-client-timeout option should not return
a delegation to the client (it should wait until an applicable answer
is found, if no entry is found in the cache).

(cherry picked from commit c3d4fd34493501140ba1bd72afba4922ba55530a)