Use swift-testing and remove tests from production

[uhooi]

Overview

• Replace XCTest with swift-testing
• Remove tests from production
• Bump target iOS from 15.0 to 16.0

Checklist

• [ ] Format code with SwiftLint (⌘B in Xcode)
• [ ] Resolve Xcode warning

References

• https://x.com/the_uhooi/status/1708449151753900232
• https://github.com/apple/swift-testing
• https://swiftpackageindex.com/apple/swift-testing/main/documentation/testing/temporarygettingstarted
• https://swiftpackageindex.com/apple/swift-testing/main/documentation/testing/migratingfromxctest
• https://github.com/apple/swift/issues/68896
• https://github.com/apple/swift-testing/issues/42