Add back one FIXME of a FN to the tests

Author: flip1995

tests/ui/use_self_trait.fixed

@@ -47,6 +47,7 @@ impl Mul for Bad {
 
 impl Clone for Bad {
     fn clone(&self) -> Self {
+        // FIXME: applicable here
         Bad
     }
 }

tests/ui/use_self_trait.rs

@@ -47,6 +47,7 @@ impl Mul for Bad {
 
 impl Clone for Bad {
     fn clone(&self) -> Self {
+        // FIXME: applicable here
         Bad
     }
 }