CBQE-2316: Update test to use mem_used stat instead of total_allocated_bytes

Change-Id: Ia8bafaa94373db13291e14e9ba04abebd4d8e253
Reviewed-on: http://review.couchbase.org/40905
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Meenakshi Goel <meenakshi.goel@globallogic.com>
Tested-by: Meenakshi Goel <meenakshi.goel@globallogic.com>