Bug 18120: /acquisitions/vendors unit tests

Bug 18120: /acquisitions/vendors unit tests

This patch introduces unit tests that need to be passed by an
/acquisitions/vendors/ REST endpoint.

To test:
- Apply the patch
- Run:
  $ sudo koha-shell kohadev
 k$ prove t/db_dependent/api/v1/acquisitions_vendors.t
=> FAIL: The endpoint is not present, should fail.

Sponsored-by: ByWater Solutions
Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>