Bug 14708: (QA follow-up) Add tests

Bug 14708: (QA follow-up) Add tests

This patch adds tests for the change. It also simplifies the delete()
method structure a bit. It fixes the error 500 the tests were raising.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail!
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass! The new behaviour (code 403) is tested!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>