Add unit tests for CensusGroup::previous_peer
Add unit tests for CensusGroup::previous_peer

Factor out previous_peer_impl to avoid the need for a full mock CensusGroup,
and modify test_census_member to take a &str rather than MemberId
(aka String) for nicer test ergonomics.

Signed-off-by: Jon Bauman <e6c6a4db54ad11b4de69fed67139594b59f3444f@users.noreply.github.com>

habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat