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>

.github/workflows/test-suite.yml

@@ -44,7 +44,7 @@ jobs:
       run: make test-release
   release-tests-windows:
     name: release-tests-windows
-    runs-on: windows-latest
+    runs-on: windows-2019
     needs: cargo-fmt
     steps:
     - uses: actions/checkout@v1
@@ -184,6 +184,16 @@ jobs:
             run: |
                 cd scripts/tests
                 ./doppelganger_protection.sh failure
+  execution-engine-integration-ubuntu:
+    name: execution-engine-integration-ubuntu
+    runs-on: ubuntu-latest
+    needs: cargo-fmt
+    steps:
+    - uses: actions/checkout@v1
+    - name: Get latest version of stable Rust
+      run: rustup update stable
+    - name: Run exec engine integration tests in release
+      run: make test-exec-engine
   check-benchmarks:
     name: check-benchmarks
     runs-on: ubuntu-latest