Add test for order=dfs with sharding (HAMT dir and multiblock file)

`TestTrustlessCarOrderAndDuplicates` introduced in #87 uses basic fixture where dir is not sharded, and every file fits in a single block.

We should add a test of `order=dfs` based on HAMT fixture such as already existing `bafybeidbclfqleg2uojchspzd4bob56dqetqjsj27gy2cq3klkkgxtpn4i` ([CAR](https://github.com/ipfs/gateway-conformance/raw/v0.3.0/fixtures/trustless_gateway_car/single-layer-hamt-with-multi-block-files.car)), to ensure ordering is respected when sharding of dirs and files is involved.