Add tests for static async functions in traits

This patch adds test cases for AFIT, the majority of which are currently
expected to run as `check-fail`.

src/test/ui/async-await/in-trait/async-example.rs

@@ -0,0 +1,17 @@
+// check-pass
+// edition: 2021
+
+#![feature(async_fn_in_trait)]
+#![allow(incomplete_features)]
+
+trait MyTrait {
+    async fn foo(&self) -> i32;
+}
+
+impl MyTrait for i32 {
+    async fn foo(&self) -> i32 {
+        *self
+    }
+}
+
+fn main() {}