Add support for Swift Testing

[dfabulich]

Skip apps use XCTest, but the new thing is Swift Testing. https://developer.apple.com/xcode/swift-testing/

Skip should transpile Swift Testing tests to JUnit and run them like XCTest tests.

[marcprux]

XCUnit will be around for a very long time to come, but support for Swift Testing is something we would definitely like to have in Skip Lite (note that it does work on the native side in Skip Fuse). Testing doesn't translate as nearly into JUnit as XCUnit, so this is a fairly large project.