tests: fix GstBus unit test with latest GLib
tests: fix GstBus unit test with latest GLib

g_source_remove() works on the default main context, and
we're doing things with a custom context. Fixes warning
with newer GLib versions.

justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer