Put all egui_demo_lib tests into their own files in their own folder (#4691)

crates/egui_demo_lib/src/demo/extra_viewport.rs

@@ -1,7 +1,7 @@
 #[derive(Default)]
 pub struct ExtraViewport {}
 
-impl super::Demo for ExtraViewport {
+impl crate::Demo for ExtraViewport {
     fn is_enabled(&self, ctx: &egui::Context) -> bool {
         !ctx.embed_viewports()
     }