How to set up a demo site?

In an issue comment @justin2pin mentioned

To enable test modules, add (or uncomment) this line in your local settings file (i.e. settings.local.php):

$settings['extension_discovery_scan_tests'] = TRUE;

So I believe that entails the following (assumes your docroot is "web"):

1. cd web/sites/default
2. cp --interactive example.settings.local.php settings.local.php
3. edit settings.local.php, search for "extension_discovery_scan_tests", and uncomment the line

However, once that's done I'm not sure how to get it to take effect.