test: add missing source for #4943

Author: cowboy-bebug

Diff for: tests/source/issue_4943.rs

@@ -0,0 +1,9 @@
+#![feature(generic_associated_types)]
+
+impl SomeStruct {
+    fn process<T>(v: T) -> <Self as GAT>::R<T>
+    where Self: GAT<R<T> = T>
+    {
+        SomeStruct::do_something(v)
+    }
+}

Diff for: tests/target/issue_4943.rs

@@ -1,3 +1,5 @@
+#![feature(generic_associated_types)]
+
 impl SomeStruct {
     fn process<T>(v: T) -> <Self as GAT>::R<T>
     where