Unified test case, use raw graphsync

[hannahhoward]

Goals

Implement the suggested refactor in #32, setup graphsync to work on raw Libp2p

Implementation

• Build a fetch method on the node interface
• Implement a node interface that uses graphsync + libp2p
• Unify all test cases to a single case with different node types

This sets us up to add experimental solutions that combine bitswap & go-graphsync by just implementing a Node interface

[adlrocha]

LGTM. Feel free to merge 🚀