Enable block_lookup_failed EF test

[paulhauner]

Description

We are currently skipping an EF test:

lighthouse/testing/ef_tests/src/cases/fork_choice.rs

Lines 159 to 164 in df51a73

	// TODO(merge): re-enable this test before production.
	// This test is skipped until we can do retrospective confirmations of the terminal
	// block after an optimistic sync.
	if self.description == "block_lookup_failed" {
	return Err(Error::SkippedKnownFailure);
	};

I haven't looked into this deeply, but I suspect that #3372 gives us what we need to re-enable this test!

CC'ing @ethDreamer since he might be interested ☺️