swift-testing: Run parameterised tests individually

[adam-fowler]

Add an option to run a swift-testing parameterised test for a version of that parameter.
I believe this will require some support from swift-testing before we can do this

[plemarquand]

For future reference this is the section of the ABI documentation in swift-testing outlining the proposal for supplying test run configuirations with JSON input, which is what we'd need to support this. It is not yet implemented, and the proposal is still in draft.