tests: fix ut_conference

The conference should be used in the tests after the first
conferenceChanged, to be sure that every calls are binded to the
conference.
Then, before adding a call to the conference, it should be CURRENT
from the host perspective, not the receiver. So, add "hostState"
to store the state of the call from the host perspective.

Change-Id: I9d27b4c0461ede05d27ea849de08c22e9fc3b249