Limit parallel test execution for cardano-testnet in flake

[palas]

Description

Testnet tests in Hydra hang often and hang in a way that prevents even restarting them until they have timed out, which takes 2 hours. This PR aims to avoid that issue by preventing cardano-testnet tests to be ran all at once, which should help in case the problem is that the RAM memory of the workers is getting full.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Runnings tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff