Rename opengl cores

[orbea]

This will rename the opengl test cores to not include libretro-test in the directory name (i.e. libretro_test_gl_compute_shaders to gl_compute_shaders). This was somewhat redundant and inconsistent with other sample cores.

Please reject this if its not desirable.

[inactive123]

we should at least have test at the beginning of the core name or something.

[orbea]

So it would be like testgl_compute_shaders? I'm fine with that, but for consistency sake should the other repo names change as well (i.e. vk_async_compute to testvk_async_compute or audio_callback to testaudio_callback)?

I'm not really that strongly for this, was just a suggestion I wanted to see if others agreed or disagreed with.