chore: Rewrite timeline redecryption tests to use HTTP mocking

This is important since we want to move the redecryption logic out of
the timeline into the main crate. This in turn means that we don't have
such low level access to the redecryption logic.

Not all tests were rewritten:
    - `test_retry_edit_and_more` Is proving to be difficult to rewrite,
      may come in a separate commit.
    - `test_retry_fetching_encryption_info` Needs verification state
      changes. Will be rewritten on the event cache layer

Author: poljar