test: show the uncurried alert during migration to [@u0]

test/blackbox-tests/uncurry-0.t

@@ -22,3 +22,26 @@ Test some error cases
                                         ^^^^^^^^^
   Error: `[@u0]' cannot be used with multiple arguments
   [2]
+
+PPX shows an alert when `@u` is used with `unit`
+
+  $ cat > x.ml <<EOF
+  > external foo : (((unit -> unit)[@u]) -> unit) -> unit = "foo"
+  > let () = foo (fun f -> f () [@u])
+  > EOF
+
+  $ melc -ppx melppx x.ml
+  File "x.ml", line 1, characters 18-30:
+  1 | external foo : (((unit -> unit)[@u]) -> unit) -> unit = "foo"
+                        ^^^^^^^^^^^^
+  Alert uncurried: This uncurried function takes a single unit argument, but will be applied with `undefined' in the compiled JS.
+  Use `[@u0]' if it is intended to have 0-arity.
+  // Generated by Melange
+  'use strict';
+  
+  
+  foo(function (f) {
+        f(undefined);
+      });
+  
+  /*  Not a pure module */