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

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Migrated from D5192 (view on Phabricator)