Unstable ep-engine test 'test_est_vb_move'

During the commit-validation builds I'm seeing test_est_vb_move fail virtually all the time with the following error:

tests/ep_test_apis.cc:11599 Test failed: `Invalid number of backfill mutations' (mutations == 3)

00:30:06.504 Aborted (core dumped)

For example during the following builds:

• http://factory.couchbase.com/job/ep-engine-gerrit-master-multi/62/label_exp=sherlocker-ubuntu12.04/console
• http://factory.couchbase.com/job/ep-engine-gerrit-master-multi/70/label_exp=sherlocker-ubuntu12.04/console

I also see it fail sometimes with a slightly different error message:

Test failed: `Invalid estimate' (estimateVBucketMove(h, h1, 0) == 8)

For example:

• http://factory.couchbase.com/job/ep-engine-gerrit-master-multi/66/label_exp=sherlocker-ubuntu12.04/console

However the test always passes fine locally on OS X and Ubuntu 14.04.

Marking this as critical as it's essentially blocking commit validation - if you get lucky you get a build though successfully, but it's something like 1-in-10.