Bug 31795: (QA follow-up) Use x-koha-override header

Bug 31795: (QA follow-up) Use x-koha-override header

Despite its title, this patch does a couple more things.

1. The tests are rewritten to cover more things, and also to avoid
   deleting all authorities inside the transaction. It is really not
   required.
2. It makes the endpoint rely on the already generically implemented
   x-koha-override header, which is intended for the same use case as
   x-confirm-not-duplicate is for.
3. It changes the return codes to match the coding guidelines [1]
4. Only checks for duplicates if no override passed.

To test:
1. Run:
   $ ktd --shell
  k$ prove t/db_dependent/api/v1/authorities.t
=> SUCCESS: Tests pass!
2. Apply this follow-up
3. Repeat 1
=> SUCCESS: Tests pass!
4. Sign off :-D

[1] https://wiki.koha-community.org/wiki/Coding_Guidelines_-_API#SWAGGER3.2.1_POST

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>