Bug 35468: Unit tests

Bug 35468: Unit tests

This patch adds all the 'basic' unit tests one would expect to find with
the api endpoints.  I've been working on this as a follow-up to bug
29002 as promised, but hadn't yet submitted them as I wanted to get the
full coverage.. but having these basics in early makes sense whilst I
continue on the more advanced cases.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 8ed228ed9e53d7616f9b2b58961d4828a3e7e8a2)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>