CBQE-0: rename _case_number -> case_number in XDCR tests

should be the same as in basetestcase

Change-Id: Ieddc3fbb6e9c511d2b9f2a6cb8d816eb47bced08
Reviewed-on: http://review.couchbase.org/30309
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>