tests: Run each test in a function

As the global object of a context is rooted, if we want the GC to act
on these objects we need to take them out of the globals.

https://bugzilla.gnome.org/show_bug.cgi?id=678737

tests/testcommon/ui.js

@@ -13,3 +13,11 @@ function init(stage) {
     let theme = new St.Theme({ application_stylesheet: stylesheetPath });
     context.set_theme(theme);
 }
+
+function main(stage) {
+    stage.show();
+    stage.connect('destroy', function() {
+        Clutter.main_quit();
+    });
+    Clutter.main();
+}