[FR] - cardano-testnet: improve command line output format

**External**

**Area**
*Other*

**Describe the feature you'd like**

We're considering replacing [Plutip](https://github.com/mlabs-haskell/plutip) with cardano-testnet, and we'd like to see some changes upstream for our convenience.

1. We'd like to make running the test suite optional, configurable via a CLI option. We really need faster startup times.
2. We'd like to see more structured output (e.g. JSON) so that we can parse the TMP dir to get the node socket path, etc.

If we implement these changes in a PR, would it be considered for acceptance?