tests/shell: Also import built-in modules as modules

The "System" module is provided by gjs itself, and also available
as module.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2836>

tests/shell/core.js

@@ -1,7 +1,7 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 /* eslint camelcase: ["error", { properties: "never", allow: ["^script_", "^malloc", "^glx", "^clutter"] }] */
 
-const System = imports.system;
+import System from 'system';
 
 const Main = imports.ui.main;
 const Scripting = imports.ui.scripting;