Use isolated PHPUnit tests and drop the dependency on Drupal Test Traits

For maintenance purposes, and ease of testing, I think it makes sense for us to switch to isolated PHPUnit tests, rather than existing-site tests supported by Drupal Test Traits. Normal tests can absolutely make sense for recipes, and would allow us to better test each recipe as a distinct component (which is how they will be distributed). We could also drop a dev dependency.

Another advantage of using isolated testing is that we will find deficiencies in each recipe; it's impossible for another recipe applied by the main Drupal CMS recipe to be filling in the gaps left by one of the components.