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 initial task here has two parts:

• add one ActivityPub-capable non-forge software to the available software instances that the test suite can deploy: Mastodon
• implement at least one federation test between Forgejo and Mastodon: Federated User Activity Following (currently WIP).

Possible follow-on tasks include:

• create API abstractions for common test actions (star, follow...) to share across different instance types
• bring in a test suite framework to better organise the test suite
• bring in a Fedi test framework (see for instance Letterbook Sandcastles)
• implement more federation tests (user following, star, ...)
• implement more federation-capable software instances (Vervis, GTS, ...)

Add Mastodon Deployment

• Use the (official) Mastodon container image as a base
• create (minimal) Mastodon configuration-settings
• create Mastodon user account etc.
• create Mastodon test data (if needed)
• solve TLS/HTTPS/plain-text

Add test for Federated User Activity Following

...