tests: Add menu item without action to gapplication

Menu items with missing actions are expected to be visible but
insensitive - add a test case for this.

tests/interactive/gapplication.js

@@ -70,6 +70,7 @@ function main() {
     section = new Gio.Menu();
     section.append('Checkbox', 'app.toggle');
     section.append('Disabled', 'app.disable');
+    section.append('Missing Action', 'app.no-action');
     menu.append_section('Subsection', section);
 
     // empty sections or submenus should be invisible