Do not use Net::DNS::Nameserver in the "serve-stale" system test

Net::DNS versions older than 0.67 respond to queries sent to a
Net::DNS::Nameserver even if its ReplyHandler returns undef.  This makes
the "serve-stale" system test fail as it takes advantage of the newer
behavior.  Since the latest Net::DNS version available with stock
RHEL/CentOS 6 packages is 0.65 and we officially support that operating
system, bin/tests/system/serve-stale/ans2/ should behave
consistently for various Net::DNS versions.  Ensure that by reworking it
so that it does not use Net::DNS::Nameserver.
10 jobs for 399-do-not-use-net-dns-nameserver-in-the-serve-stale-system-test in 8 minutes and 52 seconds (queued for 2 seconds)