Add simple impl trait test for RPITIT

tests/ui/impl-trait/in-trait/new-lowering-strategy/simple-impl-trait.rs

@@ -0,0 +1,17 @@
+// check-pass
+// compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    fn foo() -> impl Sized;
+}
+
+impl Foo for String {
+    fn foo() -> i32 {
+        22
+    }
+}
+
+fn main() {}