tests: Add a way to test cancelling introspection gets
tests: Add a way to test cancelling introspection gets

First ctrl-C in test-introspection will cancel all current and future
gupnp_service_info_get_introspection_async_full() calls.

https://bugzilla.gnome.org/show_bug.cgi?id=704867

GNOME/gupnp,GNOME/gupnp,GNOME/gupnp,GNOME/gupnp