Create a secondary package for Swift Testing integration

[Uncommon]

Because a goal for this library is to be usable for UI tests, but Swift Testing cannot be imported into UI test targets, integration with Swift Testing must be done in a separate package/module.

Things that can go in the support package:

• Conform Testing.Trait to Rundown.Trait so that ST traits can be used

[Uncommon]

Or alternatively, use package traits