Function embedder_traits::resources::resources_for_tests

fn resources_for_tests() -> Box<dyn ResourceReaderMethods + Sync + Send>

Bake all of our resources into this crate for tests, unless we are cfg!(servo_production).

Local non-production embedder builds (e.g. servoshell) can still override these with set, if runtime loading of prefs.json and other resources is needed.

In theory this can be #[cfg(servo_production)], but omitting the attribute ensures that the code is always checked by the compiler, even if it later gets optimised out as dead code.