Adjust serve-stale test

Adjust serve-stale test

The number of queries to use in the burst can be reduced, as we have
a very low fetch limit of 1.

The dig command in 'wait_for_fetchlimits()' should time out sooner as
we expect a SERVFAIL to be returned promptly.

Enabling serve-stale can be done before hitting fetch-limits. This
reduces the chance that the resolver queries time out and fetch count
is reset. The chance of that happening is already slim because
'resolver-query-timeout' is 10 seconds, but better to first let the
data become stale rather than doing that while attempting to resolve.