Add Swift Testing Compatibility

[toastersocks]

What's in this pull request?

Adds compatibility with the Swift Testing (swift-testing) framework while maintaining the existing compatibility with XCTest.

Why merge this pull request?

This would allow SwiftCheck to be used with Swift Testing.

What's worth discussing about this pull request?

I don't think so, but happy to discuss.

What downsides are there to merging this pull request?

None that I know of.