Engine API v1.0.0.alpha.6 + interop tests (#3024)

## Issue Addressed

NA

## Proposed Changes

This PR extends #3018 to address my review comments there and add automated integration tests with Geth (and other implementations, in the future).

I've also de-duplicated the "unused port" logic by creating an  `common/unused_port` crate.

## Additional Info

I'm not sure if we want to merge this PR, or update #3018 and merge that. I don't mind, I'm primarily opening this PR to make sure CI works.


Co-authored-by: Mark Mackey <mark@sigmaprime.io>

testing/execution_engine_integration/src/lib.rs

@@ -0,0 +1,12 @@
+/// This library provides integration testing between Lighthouse and other execution engines.
+///
+/// See the `tests/tests.rs` file to run tests.
+mod execution_engine;
+mod genesis_json;
+mod test_rig;
+
+pub use execution_engine::Geth;
+pub use test_rig::TestRig;
+
+/// Set to `false` to send logs to the console during tests. Logs are useful when debugging.
+const SUPPRESS_LOGS: bool = true;