tests: Add test for GLib.spawn_async_with_pipes()

https://bugzilla.gnome.org/show_bug.cgi?id=735213
Change boxed init with args to warn instead of raise

Replace raising a TypeError in gi.types.Boxed() with a warning.
Even though passing arguments or keywords to the parent class is
incorrect here, raising an exception was causing a bit of fallout
in some apps (Gramps).

https://bugzilla.gnome.org/show_bug.cgi?id=727810