test: select sink depending on running window manager

[Tropicao]

I sometimes need to switch back to X window manager. In those cases, the test script fails since the sink is configured by default to open a Wayland window, with the gfollowing logs :

Setting pipeline to PAUSED ...
Failed to set pipeline to PAUSED.
Setting pipeline to NULL ...
Freeing pipeline ...

I propose to detect the running window manager to configure proper sink. We could hardcode ximagesink in all cases, since wayland bring retrocompatibilty for X applications thanks to XWayland, but I guess it is better to use the proper sink

[stefanpartheym]

Excellent, thanks!