Fix wrong use of mempty deposit in pool registration test

[aniketd]

Description

Test failures seen on Nightly CI:

• https://github.com/IntersectMBO/cardano-ledger/actions/runs/17258631252
• https://github.com/IntersectMBO/cardano-ledger/actions/runs/17229373236

There was a stray but suspicious mempty used to compare PoolParams with an expected StakePoolState, since we do not have access to the protocol parameters in that scope. Switching to comparing just PoolParams with PoolParams makes this test actually correct.

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
• CHANGELOG.md files updated for packages with externally visible changes.
  NOTE: New section is never added with the code changes. (See RELEASING.md).
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary.
  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.

[lehins]

Perfect! Thank you.