zjsunit: Handle duplicate overrides.

We want to undo overrides in reverse order,
which is important if you override the
same name more than once in the same
function.

Until today the code basically prevented
us from ever using the original implementation
of a name we stubbed, and most of them start
as undefined due to their parent modules
starting with `set_global`.

But I do want this proper, and I introduced
a tiny pitfall today.

frontend_tests/zjsunit/namespace.js

@@ -102,6 +102,7 @@ exports.with_overrides = function (test_function) {
 
     test_function(override);
 
+    restore_callbacks.reverse();
     for (const restore_callback of restore_callbacks) {
         restore_callback();
     }