Enable tests for EEVEE Next.
As a workaround for allowing the use of EEVEE Next (still an
experimental feature) with `--factory-startup`, `arg_handle_engine_set`
enables the feature when `-E BLENDER_EEVEE_NEXT` is used.
In addition, EEVEE Next is always registered, so it's available when
calling `WM_init`.
If it's actually disabled, it will be immediately unregistered after that.
Notes:
- `get_gpu_device_type` always fails with error:
> GPU API is not available in background mode
- Setup and tests are the same as EEVEE. There are many tests that
only make sense for Cycles, and many EEVEE Next features that are
not actually tested.
Pull Request: https://projects.blender.org/blender/blender/pulls/112161
b4de568cc4