test506: make it not run in c-ares builds

As the asynch nature of it may trigger events in another order. A c-ares
upgrade made it break.

Reported-by: Marc Hörsken
Fixes #6247

tests/data/test506

@@ -55,10 +55,11 @@ run 3: overwrite cookie 1 and 4, set cookie 6 with and without tailmatch
 <server>
 http
 </server>
-# don't run this with the threaded-resolver since the events might trigger in
-# a different order!
+# don't run this with the threaded-resolver or c-ares since the events might
+# trigger in a different order!
 <features>
 !threaded-resolver
+!c-ares
 </features>
 <name>
 HTTP with shared cookie list (and dns cache)