Yeah, I'm glad to see a test suite. I've only skimmed the test, but it
seems like an excellent starting point. Initial thoughts for updating
the tests:
- Need a cmd_mode to handle the MODE command.
- Suite cmd_retr logic needs to keep dtp connection open and write a
simple header depending on MODE.

Due to the Thanksgiving holiday, I may be without network access (or
time) until next week, however.