tests: Silence valgrindlog and valgrindxml output from forked procs

This is needed to properly diff valgrind runs between commits.
Fix invalid unref after getting callable container

Don't unref the results of g_base_info_get_container() because it is
transfer-none. This was causing an attempted unref on an invalid object
when setting up callback caches for signals.

https://bugzilla.gnome.org/show_bug.cgi?id=727004