test: vaapicontext: add PLAY and NULL buttons
test: vaapicontext: add PLAY and NULL buttons

They only appear when only one sink is instanciated and their purpose
is to test the NULL-PLAY use case in context sharing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/342>

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi