Rust integration tests against the node binary #76

Putting this up to track progress on writing a test suite. Functional tests that exercise the compiled node binary end-to-end, the way floresta's python suite does, but in Rust. Use corepc-node to spawn and drive bitcoind on regtest.

• Make the node observable and drive-able from a test - test: Add integration harness for the node binary #78
• Verify the node tracks Core's chain - node: Add a getTip RPC for the chain tip #85, test: Verify the node tracks Bitcoin Core's chain #86
• Verify the node's transactions are valid to Core - test: verify the node receives a silent payment #87, test: verify the node's transaction is valid to Core #88
• Run it all in CI