Provide an easy way to access libretro test cores in development builds
In some cases (e.g. GNOME/gnome-games!197) it's useful to be able to run those cores via Games and not via retro-demo. However, doing that now involves building one of them manually, which is not ideal at all.
An example from GNOME/gnome-games!197: https://gitlab.gnome.org/GNOME/gnome-games/commit/4fff9dbf6b1a91bc51b0c184654f9aa20368778c