Revert "Comment out test_MB_12751 for now to get build going again."

Restore this test - the failure was due to build environmental issues
which have since been resolved.

This reverts commit 85581261a540335a4a53be18589272537a4176c5.

Change-Id: I4d50465920032ee01f2bacc1a99f97508adfec88
Reviewed-on: http://review.couchbase.org/44778
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>