feat: Enable mock mining in Epoch 3.0

[jbencin]

Description

Enable mock mining in 3.0 and add integration test for it

Applicable issues

• fixes Support mock mining in 3.0  #4898

Additional info (benefits, drawbacks, caveats)

Should I combine the mock_mining() integration test with follower_bootup() (it is based off that) or leave them separate?

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[obycode]

I think this looks good, just requesting some explanation on one section.