Bug 32713: Unit tests

Bug 32713: Unit tests

This patch adds unit tests to prove we return a 400 and an appropriate
error message when calling an endpoint that is not defined to support
x-koha-embed whilst passing an x-koha-embed header.

Test plan:
1) Run test prior to applying second patch and confirm it fails
2) Run test after applying second patch and confirm it passes

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 2202f7f2c29444b5cc587e09729cbf770323d96b)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>