Add additional tests and update existing tests

tests/ui/async-await/feature-async-for-loop.rs

@@ -11,4 +11,13 @@ fn f() {
     };
 }
 
+#[cfg(FALSE)]
+fn g() {
+    let _ = async {
+        for await _i in core::async_iter::from_iter(0..3) {
+            //~^ ERROR `for await` loops are experimental
+        }
+    };
+}
+
 fn main() {}