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/tests/test.rs

@@ -0,0 +1,14 @@
+use gpui::{elements::RenderElement, View, ViewContext};
+use gpui_macros::Element;
+
+#[test]
+fn test_derive_render_element() {
+    #[derive(Element)]
+    struct TestElement {}
+
+    impl RenderElement for TestElement {
+        fn render<V: View>(&mut self, _: &mut V, _: &mut ViewContext<V>) -> gpui::AnyElement<V> {
+            unimplemented!()
+        }
+    }
+}