Tests for duplicated items

So far the UPR stream responses from the fake UPR server only contained
unique mutations. With this change it's possible to return multiple
snapshots where the items *within* the snapshot is unique, but there
are duplicates across snapshot boundaries.

You can specify how many items one snapshot should have that are also
in another snapshot. The first (on-disk) snapshot won't contain duplicates.

This case happens frequently with ep-engine, but we haven't had a unit
test yet.

Change-Id: Ib54a6e9576cbca09d7208b765e04e333e413b438
Reviewed-on: http://review.couchbase.org/37125
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>