Fix timing issue in PartitionedCandidateOnStartupElection*Test

If the initial AppendEntries sent by the leader (member 1) to member 3
is delayed enough such that the behavior field in MemberActor is already
set by the test code, the AppendEntries message will be forwarded to the
Candidate behavior and yield incorrect results for the test. To prevent this,
we really shouldn't set and access the behavior field directly but instead
do so via messages to maintain actor encapsulation.

Change-Id: If497583ce648e62e3279e5abff19cb8702943c17
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>