Add system test for the deadlock fix

The test spawns 4 parallel workers that keep adding, modifying and
deleting zones, the main thread repeatedly checks wheter rndc
status responds within a reasonable period.

While environment and timing issues may affect the test, in most
test cases the deadlock that was taking place before the fix used to
trigger in less than 7 seconds in a machine with at least 2 cores.

bin/tests/system/addzone/ns3/example.db

@@ -0,0 +1,2 @@
+@ IN SOA localhost. localhost.localhost. 1 10800 3600 605800 86400
+@ IN NS localhost.