Use graceful-stop to stop httpd in CI for faster shutdown

See owasp-modsecurity#3456

Author: hnakamur

tests/run-regression-tests.pl.in

@@ -632,7 +632,7 @@ sub httpd_stop {
         -d => $opt{S},
         -f => $opt{C},
         (map { (-c => $_) } ("Listen localhost:$opt{p}", @_)),
-        -k => "stop",
+        -k => "graceful-stop",
     );
 
     my $httpd_out;