Add Mastodon in the Forgejo Federation E2E Test Suite

Goal: in the Forgejo end-to-end test suite, add Mastodon as one of the software instances, alongside Forgejo and any other supported forges.

The Forgejo end-to-end test suite performs local (self-contained) deployments and runs tests for interactions between two (or potentially more) instances: Forgejo to Forgejo, or Forgejo to Forgejo Runner, or federation between Forgejo and another ActivityPub software such as Mastodon.

The task here is:

• to add Mastodon to the available software instances that the test suite can deploy,
• to implement at least one federation test between Forgejo and Mastodon.