MB-100: add tests for 6 threads

Change-Id: I5d8cb4a5cf698509de8eed4906b440acb12e9b8e
Reviewed-on: http://review.couchbase.org/27513
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>