app: GUI: Delete root menu "Test", move its item to Demos

Its contents is a test plugin, "Test dialog".
I am responsible for it, and it is ugly, even in unstable.
Is only installed in an unstable build.

We need a convention that menu items for plugins named like "Test foo"
should only be installed in unstable build,
and should require no translations.

Note that if the Demos menu does not exist,
Gimp creates it, and it is not translated,
but only present in an unstable build.

plug-ins/python/test-dialog.py

@@ -167,7 +167,7 @@ class TestDialogPlugin (Gimp.PlugIn):
                                   "Lloyd Konneker",
                                   "2022")
         # Top level menu "Test"
-        procedure.add_menu_path ("<Image>/Test")
+        procedure.add_menu_path ("<Image>/Filters/Development/Demos")
 
         procedure.add_argument_from_property(self, "brush")
         procedure.add_argument_from_property(self, "font")