Mock tests for the comment processor module are missing

[penguineer]

The Comment Processor module introduced in #4 has only TODOs for mock tests. Add mock tests here:

• Put GitHub flow in distinct processor → already minimal
• Decide about good location for recaptcha check (form or processor?)
• TestCommentProcessor

[penguineer]

Decided to leave the recaptcha check where it is right now.
However, in a later version it might be beneficial to move it to the Processor, as recaptcha check and gettting the HEAD can be run in parallel to save on time.