Bug 25662: (follow-up) Add tests for the wrong patron_id added behaviour

Bug 25662: (follow-up) Add tests for the wrong patron_id added behaviour

This patch adds trivial tests for the new error (400) raised when the
passed patron_id doesn't exist.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> SUCCESS: Tests pass!
2. Sign off :-D

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