Bug 28370: Rewrite tests

Bug 28370: Rewrite tests

This patch rewrites the tests, and also doesn't enforce
additionalProperties to be false. We could need it, and there are routes
that need to be reviewed that would break otherwise now: error.json, for
example, is used everywhere, and some routes add some payload to it.

The main change this patch introduces is using the resolved spec instead
of finding each .json file, which wasn't even correct as we have .yaml
files already. Parameters and responses are tested.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove xt/api.t
=> SUCCESS: Tests fail! A nice report on the failures is printed

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>