Add TCP connection reset test

The TCP connection reset test starts mock UDP and TCP server which
always returns empty DNS answer with TC bit set over UDP and resets the
TCP connection after five seconds.

When tested without the fix, the DNS query to 10.53.0.2 times out and
the ns2 server hangs at shutdown.

bin/tests/system/dispatch/ns2/example.db

@@ -0,0 +1,6 @@
+example.		86400	IN	SOA	ns.example. root.example. 43 10800 900 604800 86400
+example.		86400	IN	NS	ns.example.
+ns.example.				A	10.53.0.2
+
+ns.sub.example.				A	10.53.0.3
+sub.example.				NS	ns.sub.example.