gstvalidate.supp: update location of gst.supp

Part-of: <gstreamer/gst-devtools!221>

validate/data/gstvalidate.supp

 ### This file contains either validate specific suppressions or bugs that we
 ### can't easily address because they are lower in the stack.
-### All the other suppressions should be added ton common/gst.supp
+### All the other suppressions should be added ton gstreamer/tests/check/gstreamer.supp
 
 ### Each set of suppression rules should be prefixed by either:
 ###   - FIXED: if the bug/leak has been fixed upstream but we keep the rule