feat: deflake: TestGetBlockByNumber

[arajasek]

Related Issues

This test has been observed to flake, eg. https://app.circleci.com/pipelines/github/filecoin-project/lotus/29804/workflows/e8335521-f9ab-49c3-adea-5c2252ceb830/jobs/1024823. The source of the flake is that afterNullHeight - 1 may not necessarily be a null round.

I'm a little surprised this is flaky, but it's easy enough to just check and be confident we've hit a null round.

Proposed Changes

Walk up until we are confident we've identified a null round, use that for the test's assertions.

Additional Info

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