Test: allow free vars in types for coercion.

See #6821

Free variables are not allowed in coercion, and if that happens, simple unification is performed without ever attempting coercion. This seems to go back a very long time.

There's  probably a good reason why free vars were not allowed. At the same time, there are no objects/classes supported in the language anymore, so it's not clear how those reasons would adapt.

This just marks the place where one could investigate.

Author: cristianoc

jscomp/ml/typecore.ml

@@ -2595,7 +2595,7 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg=Rejected) env sexp ty
               end else true
             in
             begin match arg.exp_desc, !self_coercion, (repr ty').desc with
-            | _ when free_variables ~env arg.exp_type = []
+            | _ when true || free_variables ~env arg.exp_type = []
                   && free_variables ~env ty' = [] ->
                 if not gen && (* first try a single coercion *)
                   let snap = snapshot () in