Enable parallel execution of tests

[carbolymer]

Changelog

- description: |
    Enable parallel execution of tests
# 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

See an explanation in similar PR in node: https://github.com/IntersectMBO/cardano-node/pull/5692

Before change:

All 204 tests passed (16.75s)
Test suite cardano-cli-golden: PASS
...
All 22 tests passed (3.94s)
Test suite cardano-cli-test: PASS

After change

All 204 tests passed (12.38s)
Test suite cardano-cli-golden: PASS
...
All 22 tests passed (1.97s)
Test suite cardano-cli-test: PASS

Checklist

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