Additional unit test code.
Do not introduce incomplete candidates.

Since we are only able to assess either the LAN or the WAN address of a
candidate, it is possible that the other address remains 0.0.0.0:0 (if
that is what the candidate reported).  These candidates should not
introduced
A Candidate.wan_address may overlap with ours.

it is possible that the WAN address that someone claims to have is
correct, while the WAN address that we believe we have is not.  it is
not a problem to have candidates with the same WAN address as us.