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.

crates/gpui_macros/Cargo.toml

@@ -13,6 +13,3 @@ doctest = false
 syn = "1.0"
 quote = "1.0"
 proc-macro2 = "1.0"
-
-[dev-dependencies]
-gpui = { path = "../gpui" }