Beginnings of private subdomain test.

See also https://codereview.appspot.com/110690043/.

@eranmes
Sleeps in tests are considered bad form because it is not clear what is the explicit criteria for the continuation of the test. Also, when the timeout is not enough, it's not clear what would break.
I see two ways around this:
(1) Make the ct-rfc-server output something when it's ready and continually read its stdout until you see it.
(2) Continually call a probe that will try to connect to the server, until it succeeds.