Feature: Add test for adding extra configs in CI

[jessebot]

Description of the change

We should add a test for setting extra Nextcloud values via nextcloud.configs. We should set something via an inline php file using the helm parameter, and then verify it got set with occ config:system:get on the pod after the ct install is done.

Benefits

It's really common for users to need to add extra configuration config.php files, so this would ensure it's working.

Possible drawbacks

none that I can think of, but always open to feedback :)