swh.auth.pytest_plugin: Make decoded_token consistent with user_info

Tester will want to craft carefully the keycloak mock so it returns consistent data.

Currently the decoded_token and user_info are hard-coded to consistent values. So using the default keycloak mock is ok. As soon as other testers will want another user_info, the decoded token will diverge and not return the correct user_info subpart of the decoded token.

This commits fixes such behavior and allows the decoded token to stay consistent with the user_info.

Related to swh-deposit#2858 (closed)

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Migrated from D5230 (view on Phabricator)