'L{StringListMailbox.listMessages} returns the length of the message at
the offset into the mailbox passed to it.'

def test_listOneMessage(self):

    mailbox = mail.maildir.StringListMailbox(['abc', 'ab', 'a'])
    self.assertEqual(mailbox.listMessages(0), 3)
    self.assertEqual(mailbox.listMessages(1), 2)
    self.assertEqual(mailbox.listMessages(2), 1)
