Don't cleanup the dns_message_checksig fuzzer in atexit handler

Don't cleanup the dns_message_checksig fuzzer in atexit handler

After the dns_badcache refactoring, the dns_badcache_destroy() would
call call_rcu().  The dns_message_checksig cleanup which calls
dns_view_detach() happens in the atexit handler, so there might be
call_rcu threads started very late in the process.  The liburcu
registers library destructor that destroys the data structured internal
to liburcu and this clashes with the call_rcu thread that just got
started in the atexit() handler causing either (depending on timing):

 - a normal run
 - a straight segfault
 - an assertion failure from liburcu

Instead of trying to cleanup the dns_message_checksig unit, ignore the
leaked memory as we do with all the other fuzzing tests.