test: show wrong "unused attribute" warning for @@@mel.config in .mli

test/blackbox-tests/unused-attributes.t

@@ -17,3 +17,16 @@
 
   // Generated by Melange
   /* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
+
+
+  $ cat > x.mli <<EOF
+  >   [@@@mel.config { flags = [|"-unboxed-types"|] }]
+  > EOF
+  $ melc -ppx melppx x.mli
+  File "x.mli", line 1, characters 6-16:
+  1 |   [@@@mel.config { flags = [|"-unboxed-types"|] }]
+            ^^^^^^^^^^
+  Alert unused: Unused attribute [@mel.config]
+  This means such annotation is not annotated properly.
+  For example, some annotations are only meaningful in externals
+