various tests: do not provoke SIGTRAP with -m no-undefined

various tests: do not provoke SIGTRAP with -m no-undefined

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