create-testnet-data: fix that treasury could be unexpectedly negative

[smelc]

Changelog

- description: |
    create-testnet-data: fix that treasury could be unexpectedly negative
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  - improvement    # QoL changes e.g. refactoring
  - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Fixes https://github.com/IntersectMBO/cardano-cli/issues/643

How to trust this PR

I wanted to test that cabal test cardano-testnet passed on cardano-node, using this version of cardano-cli; but it's not easy to do right now. Still I think this is safe.

Checklist

• [X] Commit sequence broadly makes sense and commits have useful messages
• [X] New tests are added if needed and existing tests are updated. See Running tests for more details
• [X] Self-reviewed the diff