fix: test: deflake TestQuotePriceForUnsealedRetrieval

[arajasek]

Related Issues

This test failed flakily here.

The underlying issue is that the (only) miner on the network was busy sealing a sector, and so missed its PoSt. That led to it being slashed, which halts the network since the power is 0. The test eventually times out.

Proposed Changes

The bad news is that this issue affects many tests. The good news is that @ZenGround0 already has a fix, which is the BeginMiningMustPost method. We just need to use this method!

Additional Info

Checklist

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