[test scenario] Sectors expire on-time

[Stebalien]

Describe the test scenario.

Verify that sectors expire on-time (immediately after the end of the last challenge window after their expiration).

1. Commit two sectors, one that expires soon, one that expires later.
2. Wait for the first sector to expire.
3. Validate.

What should we measure?

1. The miner should not be penalized for anything.
2. Lotus should continue proving the expired sector until it's actually terminated on-chain.
3. The sector state should switch to terminated (and proofs should stop).
4. Proofs for other sectors within the same deadline should continue to work.
5. Pledge collateral should be unlocked.

Discomfort factor (0-10).

4 only because the minimum sector lifetime is 6 months. We have time to fix this.