Add mechanism to change Weston's behavior when running certain tests
This is to address #422 (closed). As it will benefit both !453 and !458 (merged), I think it is a good idea to open this MR to unify the efforts. We can use it to propose and discuss a solution that is acceptable in the long term and also that works for both use cases.
There are some tests that require the compositor to have a different behavior than the default. For instance, in !453 we need to be able to use a fake time clock in certain tests. In !458 (merged) we want to disable the behavior of falling back to a renderer screenshooter if we want to use a test that depends on the writeback screenshooter.
For now it is just a prototype, any suggestions are welcome. Here we have it being used in !458 (merged):
https://gitlab.freedesktop.org/leandrohrb/weston/-/blob/screenshot/libweston/backend-drm/drm.c#L2713
Closes: #422 (closed)