Add tests

[jsdw]

This library has no tests currently. This isn't quite as bad as it seems because it's been tested fairly extensively via https://github.com/jsdw/polkadot-historic-decoding-example (so we can see that it succeeds in decoding extrinsics and storage entries across a bunch of runtimes and blocks) and via its integration PR into Subxt (paritytech/subxt#1785), both of which are helping to turn up any issues.

That said, it would be good to add some tests! A couple of options:

• Store more extrinsic hex's in artifacts with the corresponding metadatas, and manually ensure that they decode as expected (same with storage entries). This is fairly verbose and doesn't cover as much ground as the historic decoding example, but helps prevent us from breaking things in isolation going forwards.
• Have some more E2E thing or integrate a simple version of the historic decode stuff to point at some RPC node and try decoding some set of blocks? Easier than the above in the sense that we don't need to save a bunch of offline data, but more brittle if RPC servers have issues and requires pulling an RPC server etc in.
• Manually build extrinsics via sp_core etc?