Similar to what's already present in RTPEM_Emulation, this allows tests
to retrieve the Osmux messages received.
Related: OS#5987
Change-Id: Id9aa3a0a02ef5a5e39b4df8a1c165f35255829ab
If test calls RTPEM_bind twice, the previous socket is kept open
(ConnId 1) while the new one is assigned to the the expected ConnId for
RTP/RTCP packets received (ConnId), however, if remote was already
sending packets, it may happen that the port still receives those with
ConnId=1, which may make test fail with message:
"Received unexpected type from RTP"
Change-Id: I73f4af4e590dd3958e3f4d1dba0496c0750d642d