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.

bin/tests/system/conf.sh.common

@@ -114,6 +114,7 @@ rrsetorder
 rsabigexponent
 runtime
 sfcache
+shutdown
 smartsign
 sortlist
 spf