Add option to print filenames in GDScript unit testing

[anvilfolk]

Allows using the --print-filenames option when running or generating GDScript tests. This should make it easier to identify which file is causing the segfault with whatever new amazing feature the GDScript folks are implementing :)

Also removed the legacy test option --gdscript-test.

[Calinou]

Remember to update the Unit testing documentation in a PR to godot-docs :slightly_smiling_face:

[anvilfolk]

@Calinou I forked the docs to get on it - just want to make sure this gets approved. I don't have a great track record lately with get PRs merged ;D

[akien-mga]

Thanks!