Write integration tests for time-based tenure extensions

[obycode]

Designing good integration tests for this new behavior is important. We will need to test several different scenarios:

• Extend the tenure after all signers report idle timestamps which have passed (active mining)
• Extend the tenure after estimated timestamp has passed (idle miner)
• Integration test: Extend the tenure when < 70% of signers have reached their idle time #5547
• Validate that STX transfer blocks do not count against the signer's downtime calculation

See #5434