Fix member visibility in tests

$modules should be protected. This was true in D8 (and D7 for that matter) but will now be enforced in D9 since $modules is now a protected property in tests

Likewise, setUp() should be protected. One of the tests does this, the other does not. See https://api.drupal.org/api/drupal/core%21tests%21Drupal%21Tests%21Browse...