Improve gsd-power test runtime
It should be easy to improve the runtime by simply running multiple gsd-power tests in parallel. In general, this could work by simply moving the tests into multiple subclasses and starting separate runs for each of these classes from meson.
There are only two issues that block such a change currently:
- Mock brightness is a fixed file (will be fixed by !14 (merged))
- Mock external monitor is a fixed file (needs work)
A simple environment variable to indicate which file should be used could be sufficient here.