MB-20852 [1/N]: Update tests to facilitate set_vbucket_state changes

MB-20852 [1/N]: Update tests to facilitate set_vbucket_state changes

Use checkXX() macros instead of cb_assert() in a number of test which
check for number of items enqueued.

Also adjust the tests to find the initial value of ep_total_enqueued
before storing items, and use this when checking for expected
values. This facilitates the tests working once the changes for
MB-20852 add the set_vbucket_state meta-item to disk queues.

Change-Id: I0108cc5b635b5376d5852b513a6ed47e67f1e62b
Reviewed-on: http://review.couchbase.org/69010
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>