DELEG Imp spec

[teodanciu]

Description

Imp tests for DELEG rule, testing various scenarios of delegating stake, votes and both.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• 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)
• 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)
• 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]

This looks really good. Let's wait for #4555 be merged, since this PR depends on it.

[teodanciu]

This is now adjusted to the fix for the unexpected deposit failure when unregistering a non-registered credential, that has been merged to master.

[lehins]

Looks great. Thank you!