tests for LUSERS

[slingamn]

ergochat/ergo#286, ergochat/ergo#814, ergochat/ergo#848, ergochat/ergo#1077, ergochat/ergo#1225, ergochat/ergo#1303, ergochat/ergo#1326

Probably a lot of these could be caught by basic testing. Some cases:

1. Add and remove an anonymous user without the +i mode as default
2. Add and remove an anonymous user with the +i mode as default
3. Add and remove a single session of an always-on client
4. Test the "unknown" (unregistered connection) count in the middle of a registration
5. Test disconnecting without completing the registration

[slingamn]

This was begun but I don't know which line items were completed: https://github.com/oragono/irctest/blob/master/irctest/server_tests/test_lusers.py