cardano-testnet: Clean up conf constructor naming in cardano-testnet

[palas]

Description

Testnet.Start.Types had three constructors for *Conf* whose names did not communicate how they differ: mkConf, mkConfig and mkConfigAbs. See issue: #6569

This PR:

• Removes mkConf (Its only difference with the pure mkConfig was noting the directory in the test log, which was redundant, since every call site passes a workspace path that hedgehog-extras workspace already annotates it on creation ("Workspace: ").
• Test code now uses mkConfig directly, which is now exported (and re-exported from Cardano.Testnet in place of mkConf). The call sites that additionally wrapped mkConf in H.noteShowM keep their Conf annotation via H.noteShow.
• Renames mkConfigAbs to mkConfigAbsolute, and documents that, besides making the path absolute, it also creates the directory if it does not exist yet.

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 Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
  • cardano-node-chairman, cardano-submit-api and cardano-testnet instead need a
    changelog fragment in <package>/.changes/, because their CHANGELOG.md is generated
    from fragments at release time. Copy _TEMPLATE.yml from that directory, or run
    nix run github:input-output-hk/cardano-dev#herald -- new
• 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