'Create empty maildir \'folder\' in our test maildir
Does not fail if it already exists'

@classmethod
def create_maildir(cls, folder):

    assert (cls.testdir != None)
    maildir = os.path.join(cls.testdir, 'mail', folder)
    for subdir in ('', 'tmp', 'cur', 'new'):
        try:
            os.makedirs(os.path.join(maildir, subdir))
        except OSError as e:
            if (e.errno != 17):
                raise
