Clean up mkConf / mkConfig / mkConfigAbs naming in cardano-testnet

`Testnet.Start.Types` has three functions for creating `Conf` values:

- `mkConf :: (HasCallStack, MonadTest m) => FilePath -> m Conf` - annotates the path in test output, then calls `mkConfig`
- `mkConfig :: FilePath -> Conf` - pure, adds trailing separator and defaults
- `mkConfigAbs :: FilePath -> IO Conf` - makes path absolute, creates directory, calls `mkConfig`

The names do not communicate the differences.
`mkConf` vs `mkConfig` looks like a typo rather than a meaningful distinction.
`mkConfigAbs` conflates "make path absolute" with "run in IO".

Suggested cleanup (open to better ideas):
- `mkConfig` stays as the pure core
- `mkConf` becomes something like `mkConfigForTest` or `mkConfigAnnotated` to signal the `MonadTest` dependency
- `mkConfigAbs` becomes something like `mkConfigAbsolute` or `mkConfigIO`

Or collapse them if the separate variants are not pulling their weight.

**Context:** Flagged by @carbolymer in https://github.com/IntersectMBO/cardano-node/pull/6346 review comments.