Silence a Perl warning output by stop.pl

On Unix systems, the CYGWIN environment variable is not set at all when
BIND system tests are run.  If a named instance crashes on shutdown or
otherwise fails to clean up its pidfile and the CYGWIN environment
variable is not set, stop.pl will print an uninitialized value warning
on standard error.  Prevent this by using defined().

bin/tests/system/stop.pl

@@ -240,7 +240,7 @@ sub clean_pid_file {
 	if (send_signal(0, $pid) == 0) {
 		# XXX: on windows this is likely to result in a
 		# false positive, so don't bother reporting the error.
-		if ($ENV{'CYGWIN'} eq "") {
+		if (!defined($ENV{'CYGWIN'})) {
 			print "I:$test:$server crashed on shutdown\n";
 			$errors = 1;
 		}