ensured tests use the spawned instance

this not only ensures that the instance works as expected, it also
simplifies the setup considerably

caveats:

* not sure why the "templates" symlink is required
* it appears the instance's `tiddlywebconfig.py` is not being evaluated;
  within `load_app` the settings from `instance.instance_config` (e.g.
  `server_host`, `system_plugins`) are no longer present, even when
  forcing a `merge_config` beforehand

/cc @cdent