I believe the tests passed with the bug present.  My fix was specifically 
in MailGW.do_mailbox - that is, the input method for one particular way of 
putting received emails into Roundup (the other methods had smaller fixes 
to use message_from_bytes / message_from_binary_file, although POP and 
IMAP could do with testing that they actually work with Python 3).  The 
tests in test_mailgw.py don't test the external input methods, just the 
MailGW.main method, and MailGW.main was calling email.message_from_file 
with RoundupMessage passed as the second argument before my patch (so 
probably unaffected for Python 2 but probably having Python 3 issues with 
mixed character set 8-bit-encoded attachments before my patch).

Generally the tests operate at the Python API level and are weak on 
external input/output interfaces (which is also an area where Python 3 
issues are likely and so manual testing for those is important).