genesis creation: share code

[smelc]

Changelog

- description: |
    Share code between `genesis` commands and `create-testnet-data`
# 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
  - refactoring    # QoL changes
  # - 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

1. This is a small QoL improvement on its own.
2. And it's further built upon by create-testnet-data: create byron genesis #974

How to trust this PR

• I have run cardano-testnet tests with this PR.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff