Bug 20854: Add more CAS tests

Bug 20854: Add more CAS tests

This patch aims to add full test coverage for 'logout_cas' the way we do
it nowadays (in a subtest, mocking things, etc). It is provided as a
separate patch so we can test the follow-up tidy and see no behavior
change.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Auth_with_cas.t
=> SUCCESS: Tests pass!
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass!
4. Apply the refactoring follow-up
5. Repeat 3
=> SUCCESS: Tests still pass!
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>