Bug 1787182 [Linux] Run VA-API tests on supported hardware only r=emilio,gfx-reviewers,jgilbert

- Implement fire_vaapi_process() which launch VA-API test utility on given DRM device.
- Implement GfxInfo::GetDataVAAPI() which gets VA-API test results
- Run VA-API tests when FEATURE_HARDWARE_VIDEO_DECODING is probed and only if it's enabled by GfxInfo.

Differential Revision: https://phabricator.services.mozilla.com/D171995