CBQE-0: refactor py-xdcr-unidirectional

Some scenarios have the same verification or overlap

Change-Id: Ib1f79855a65ccc7307bd68f3067a5298738aab72
Reviewed-on: http://review.couchbase.org/32990
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>