Add system test for the deadlock fix

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.