improve tests for preserve_expiry
improve tests for preserve_expiry

in order to find out why CI might fail on these tests from time to time,
the test cases updated with more assertions and additional CAS checks

Change-Id: I0b514ea264d1c55a93453d15e2d7fe329939009e
Reviewed-on: http://review.couchbase.org/c/libcouchbase/+/160410
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>

couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase