Even though we can't always make no-leak guarantees when g_warning()
in this case we're testing this behavior in tests, and it would be
good to be able to valgrind this.
https://bugzilla.gnome.org/show_bug.cgi?id=711751
Some of the GLib tests deliberately provoke warnings (or even fatal
errors) in a forked child. Normally, this is fine, but under valgrind
it's somewhat undesirable. We do want to follow fork(), so we can check
for leaks in child processes that exit gracefully; but we don't want to
be told about "leaks" in processes that are crashing, because there'd
be no point in cleaning those up anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=666116
