Comment out test_MB_12751 for now to get build going again.

Change-Id: Ic3a25a28c2a61c58a24165ee02b5205063a6a6e6
Reviewed-on: http://review.couchbase.org/44448
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: buildbot <build@couchbase.com>