swh.auth: Add coverage to the decode_token endpoint
This reworks the tests logic as well to reuse the way web tests are written.
This is a first step to actually use the mock class defined here as fixture for future modules which will depend on swh-auth (swh-web-client, swh-web and swh-deposit).
Related to T3079
Migrated from D5192 (view on Phabricator)