Add tests for file sorter callback and skip_writeback option

Change-Id: I87246cf28fdb3b524d219d7f92c82ffb425c4326
Reviewed-on: http://review.couchbase.org/32660
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>