v8+ conformance tests

[Stebalien]

Currently, we pull in the v7 bundle via cargo. However:

1. We no longer publish the bundle to crates.io.
2. We now build the bundle in github CI.

So, we should:

1. Remove the v7 conformance tests.
2. Extract conformance tests for v8 (first from calibrationnet, then from mainnet).
3. Make the conformance tests download the bundle from GitHub.

This should also significantly improve performance of the conformance tests.