Import DAG reader tests involving `NodePromise`s to actually test `NavigableIPLDNode`

The node prefetching mechanism has been abstracted away from the DAG reader in `go-unixfs` into the `NavigableIPLDNode` here, so the PR https://github.com/ipfs/go-unixfs/pull/12 which employs the `Walker` (that relies on `NavigableIPLDNode`) had to remove the related tests that acted directly on the internal `NodePromise` structure,

https://github.com/ipfs/go-unixfs/pull/12/files#diff-5a645da404e1845b770b90d1c81e71c6

Those tests should be recreated here (possibly without involving the DAG reader now, just testing the `FetchChild` and related methods of `NavigableIPLDNode` to actually create the `NodePromise`s following the spirit of the original test).