MB-10273 Add tests for file merger with deduplication

Change-Id: I26618c6fd5f02c9b8b79ff9377315ef19e620bfd
Reviewed-on: http://review.couchbase.org/34982
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>