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.

cablelabs/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,lubosz/gstreamer,lubosz/gstreamer,lubosz/gstreamer,surround-io/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,lubosz/gstreamer,ensonic/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,cablelabs/gstreamer,centricular/gstreamer,centricular/gstreamer,mparis/gstreamer,mparis/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,centricular/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,krichter722/gstreamer,mparis/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,krichter722/gstreamer,centricular/gstreamer,cfoch/gstreamer,cfoch/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,krichter722/gstreamer