Add unittests for SMTP protocol responses of b.e.s.MailFactory.

These tests emulate, in Twisted, what the `telnet-an-email` [0] script
did by speaking raw SMTP over telnet in an embarrassing pexpect script
which was never committed for fear being chained in stocks and having
rotten tomatoes thrown at me.

Instead of that telnet nonsense, these tests use mocked `ITransport`s
from `twisted.test.proto_helpers` to create a transport layer for
sending SMTP protocol commands.

 * FIXES part of #9874 by automating some manual portions of the steps
   required for testing BridgeDB's email distributor.

[0]: https://trac.torproject.org/projects/tor/attachment/ticket/5232/telnet-an-email