Simplify TCPAcceptTest by using fchown.

When this code was written, we didn't know that it was possible
to change the UID of a socket using fchown(). As a result, the
UID routing codepath created the listen socket in a different
way and on a different port than self.listenport, which is
confusing.

Change-Id: Ia8e47ce54a5780433bbf413b8966b7912b43135c