Provide an easy way to access libretro test cores in development builds
In some cases (e.g. !197 (merged)) 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 !197 (merged): https://gitlab.gnome.org/GNOME/gnome-games/commit/4fff9dbf6b1a91bc51b0c184654f9aa20368778c