Refactor LeaderTest

Refactor LeaderTest

Removed the remaining Within blocks in tests.

Changed each test to use the leaderActor and followerActor class members for
consistency. The actors are killed on tearDown. The creation of a
JavaTestKit block is no longer necessary and was removed from each test.

Added a shorcut method createActorContextWithFollower that creates the
leader context and adds a peer follower. This eliminates some duplicate code
in the tests.

Added an expectFirstMatching method to MessageCollector that waits for
the message in contrast to getFirstMatching.

Added some more assertions in some tests.

Change-Id: Id4ee0291352999b31e40abd9895e3d0237acf432
Signed-off-by: tpantelis <tpanteli@brocade.com>