Treasury withdrawals tests

[lehins]

Description

This PR reorganized the ImpTest framework a little. Gives it a bit more structure.

Adds ability to create reward accounts in ImpTest

Adds tests for TreasuryWithdrawals that replicate #3827 bug and confirm #3831 fix

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[TimSheard]

Not too familiar with the framework, but the new code has more structure, so that is good.