Fix sporadic failure in telnet tx tests

We occasionally get tests failing like so:

```
Traceback (most recent call last):
  File "/Users/jerith/.virtualenvs/vumi/lib/python2.7/site-packages/twisted/internet/defer.py", line 1070, in _inlineCallbacks
    result = g.send(result)
  File "/Users/jerith/code/vumi/vumi/transports/telnet/tests/test_telnet.py", line 182, in test_transport_type_override
    self.assertEqual(msg['transport_type'], 'foo')
  File "/Users/jerith/.virtualenvs/vumi/lib/python2.7/site-packages/twisted/trial/_synctest.py", line 356, in assertEqual
    % (msg, pformat(first), pformat(second)))
twisted.trial.unittest.FailTest: not equal:
a = u'telnet'
b = 'foo'
```