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/ans.pl should behave
consistently for various Net::DNS versions.  Ensure that by reworking it
so that it does not use Net::DNS::Nameserver.

(cherry picked from commit c4209418)
7 jobs for 399-do-not-use-net-dns-nameserver-in-the-serve-stale-system-test-v9_12 in 6 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #23119
amd64 docker linux

00:00:10

 
  Build
passed build:debian:sid:amd64 #23120
amd64 docker linux

00:01:48

passed build:debian:sid:i386 #23121
i386 docker linux

00:01:51

 
  Test
passed systemtest:debian:sid:amd64 #23124
amd64 docker linux

00:04:35

passed systemtest:debian:sid:i386 #23125
i386 docker linux

00:04:38

passed unittest:debian:sid:amd64 #23122
amd64 docker linux

00:00:51

passed unittest:debian:sid:i386 #23123
i386 docker linux

00:00:52