Test: Governance rollback tests

[sgillespie]

Description

Addresses #1572. Add some more Conway tests:

• rollback new committee member (enacted)
• rollback new committee proposal (not ratified)
• chained new committee proposals
• rollback hard fork

Checklist

• Commit sequence broadly makes sense
• Commits have useful messages
• New tests are added if needed and existing tests are updated
• Any changes are noted in the changelog
• Code is formatted with fourmolu on version 0.10.1.0 (which can be run with scripts/fourmolize.sh)
• Self-reviewed the diff

Migrations

• The pr causes a breaking change of type a,b or c
• If there is a breaking change, the pr includes a database migration and/or a fix process for old values, so that upgrade is possible
• Resyncing and running the migrations provided will result in the same database semantically

If there is a breaking change, especially a big one, please add a justification here. Please elaborate
more what the migration achieves, what it cannot achieve or why a migration is not possible.

[Cmdv]

From my limited understanding of Governance these tests seem correct to me. One very minor comment that can be ignored.

🚢

[Cmdv]

very minor but is it worth abstracting this hash out as it's used in 2 places? Happy to leave as is tbh.