self.assertEqual(field_in_passwd(user_mod.name,6),myshell)

        self.assertEqual(field_in_passwd(user_mod.name,5),myhome)

Magic numbers.

    def test_try_to_create_badass_user(self):

Test for the inverse set of characters would be nice. This asserts, that *one char* usernames are accepted.

        badchar = [chr(i) for i in range(1,36) + range(37,48) + range(58,65) + range(91,95) + range(96,97) + range(123,127)]

These magic numbers are not well readable. Maybe you could use something like:
badchar = [chr(i) for i in xrange(0, 128) if chr(i) not in set(string.ascii_letters + string.digits + '$_')]

            self.assertEqual(user.rval, -1)

If an assertion fails, examiner has to test each character from the badchar set until he finds out, which one caused it. I'd recommend add some description string like:
self.assertEqual(user.rval, -1, 'account with invalid username "%s" created')