tests: Destroy GjsContext before exit

This will be required in the upcoming version of GJS.

https://bugzilla.gnome.org/show_bug.cgi?id=775374

src/run-js-test.c

@@ -127,6 +127,7 @@ main(int argc, char **argv)
   gjs_context_gc (js_context);
   gjs_context_gc (js_context);
 
+  g_object_unref (js_context);
   g_free (script);
   exit (code);
 }