Bug 20624: Unit tests

Bug 20624: Unit tests

This patch adds tests to verify that disabling the RESTOAuth2ClientCredentials syspref
makes any request on the /api/v1/oauth/token using the
'client_credentials' grant fail with 'grant not implemented'.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/oauth.t
=> FAIL: Tests fail because the change is not implemented!

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>