refactor: Move tests to their own files

- This [is said to lead improve compilation
  speed](https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html#Assorted-Tricks)
- When grepping for a function invocation, this makes it easy to see whether it's from a test or "real" code
- We're calling the files e.g. `chat_tests.rs` instead of `tests.rs` for the same reason why we moved `imap/mod.rs` to `imap.rs`: Otherwise, your editor always shows you that you're in the file `tests.rs` and you don't know which one.

This is only moving mimeparser and chat tests, because these were the
biggest files; we can move more files in subsequent PRs if we like it.