Enable and fix SMTPUTF8SimTests in test_smtplib.

The tests for SMTPUTF8SimTests in test_smtplib.py were not actually
being run because test_smtplib was still using the 'test_main' pattern,
and the class was never added to test_main.

Additionally, one of the tests needed to be moved to the non-UTF8 server
class because it relies on the server not being UTF-8 compatible (and it
had a bug in in).