Convert SsoUserDataAlterTest from a functional test to a kernel test

This is currently implemented as a functional test, which is quite slow compared to a kernel test, and given that we don't need to access the site as a user, it's arguably overkill.

https://github.com/neurocracy/drupal-omnipedia-discourse/blob/6bf1887e9a631a7f9687c2f2ca0414658e36cc1c/tests/src/Functional/SsoUserDataAlterTest.php#L21