Move gpui derives tests into gpui crate to avoid dependency cycles

`cargo run` on Zed project leads to rust-analyzer evantually emitting

`[ERROR project_model::workspace] cyclic deps:
gpui_macros(Idx::<CrateData>(269)) -> gpui(Idx::<CrateData>(264)),
alternative path: gpui(Idx::<CrateData>(264)) ->
gpui_macros(Idx::<CrateData>(269))`

error after loading the project.

The PR fixes this by moving away the test to the "root" project.