Enable tests exercising LMD GHOST fork choice

[ralexstokes]

Tests added in ethereum/trinity#1833 are currently disabled as they timeout in CI.

The tests run locally and while they are markedly slower, they don't seem to be taking as long as CI reports (exceeding the 10min context deadline...).

These tests are high-level and arguably are "integration tests" -- Does something change if we run the tests in their own CI run? (an eth2-integration test run???)

Otherwise, we should implement performance enhancements to the state transition and re-enable them.

[ralexstokes]

For what it is worth, the tests do run and pass locally...

[ralexstokes]

The tests do run faster locally but still timeout in CI (I'm assuming the CI environment does not give a lot of compute cycles, relatively speaking...)