feat: deflake splitstore_test

[arajasek]

Related Issues

This test has been observed to flake, example here. In this case, the test ran forever because the Prune couldn't start at the time it was triggered, and so we just waited endlessly for a prune that was never going to happen.

Proposed Changes

• Do NOT mine while waiting for pruning to occur (I think by this point we don't need the chain to progress)
• Re-send the Prune request every 5 seconds if pruning doesn't start for some reason

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[arajasek]

Would get superseded / rendered unnecessary by #10840