Added test for the fix

This test ensures that named will correctly shutdown
when receiving multiple control connections after processing
of either "rncd stop" or "kill -SIGTERM" commands.

Before the fix, named was crashing due to a race condition happening
between two threads, one running shutdown logic in named/server.c
and other handling control logic in controlconf.c.

This test tries to reproduce the above scenario by issuing multiple
queries to a target named instance, issuing either rndc stop or kill
-SIGTERM command to the same named instance, then starting multiple rndc
status connections to ensure it is not crashing anymore.

(cherry picked from commit 042e509753)

bin/tests/system/shutdown/ns2/test.db

@@ -0,0 +1,7 @@
+$TTL 300
+
+@	IN	SOA	ns1 root.test. 2020040101 4h 1h 1w 60
+@	IN	NS	ns1
+ns1	IN	A	10.53.0.2
+@	IN	A	10.53.0.2
+www	IN	A	10.53.0.2