Added UnitTestTools and IncrementalStakeTest

[TimSheard]

This is a second try at addressing #4171.
It replaces PR #4211, which has been abandoned.

Issue #4171 identified some potential problems with the code in [eras/shelley/impl/src/Cardano/Ledger/Shelley/LedgerState/IncrementalStake.hs]. It suggested some tests to resolve the issue. Such a test need specific values with types such as Credential, StakeReference, UtxO, IncrementalStake, PParams, Rewards, Deposits, all with complex interrelation-ships.

Test.Cardano.Ledger.UnitTestTools makes it easy to express such relationships
Test.Cardano.Ledger.Shelley.UnitTests.IncrementalStakeTest exhibits a test, that resolves the issue.

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

[lehins]

The test looks awesome!

As usual I have a bunch of suggestions, but none of them are about the test case. Hopefully you'll like my suggestion to improve the tuple solution 😁

[lehins]

Beautiful.
Thank you for your patience, Tim!

[lehins]

Versioning is off.

[lehins]

Perfect