[BUG] Testground testkit devnet sometimes starts miners on different chains

[nonsense]

Describe the bug
The Testground Lotus testkit used in the lotus-soup testplan is not starting devnets correctly. Occasionally miners start mining on different chains, resulting in flaky tests. This should be fixed if we want to enable more elaborate Testground Lotus tests, such as deals end-to-end tests or payment channel tests in networks of more than 1 miner.

Version (run lotus version):
4688da51

To Reproduce
Steps to reproduce the behavior:

1. Run the Testground payment channel stress tests (part of CI) with multiple miners, instead of just 1.
2. See error: failed to sync fork: synced chain forked at genesis, refusing to sync

Expected behavior
The lotus-soup testkit should be able to consistently start devnets with multiple miners (2, 3, 5, etc.)

[BigLep]

@jennijuju : I don't think we should say this is in review. #6126 is a mitigation so we can get testground running but this issue is about fixing underlying issues. I would link #6126 to some of the enable testground tests issues and this issue still needs to get prioritized.

@nonsense : let me know if I have any of that wrong.

[magik6k]

If those miners have the same power, this might be related to #2981