add unit test for max_threads override feature

By default, ep-engine will spawn 2X times the number of cpu
cores, threads. But we also have an way to override this
by specifying a non-zero max_threads parameter.
Enhance existing unit test to test this feature too

Change-Id: I1f8e0a680bb79f0883a86e763fb13bcaab6ee6e4
Reviewed-on: http://review.couchbase.org/31384
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>