Bug 20624: (QA follow-up) Unit tests for missing deps situation

Bug 20624: (QA follow-up) Unit tests for missing deps situation

This patch tests the situation in which Net::OAuth2::AuthorizationServer
is missing. It mocks Module::Load::Conditional::can_load and expects the
/token endpoint answers 'Unimplemented grant type' to all requests, and
the 'authenticate_api_request' in 'under' exit with unauthorized (403)
to requests in which the Authorization header is passed containing a
Bearer token, but OAuth2 is not really available.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/oauth.t
=> FAIL: Tests fail because our REST endpoints don't support this
behaviour.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>