[Pitch] Declaring trait configurations for test targets for easier testing

Hello Swift community! I'd like to pitch an idea that lets you easily test your test targets across different trait configurations in your package. Traits have been a great recent addition to the Swift Package Manager, but when it comes to running unit tests that rely on non-default trait configurations, you'll end up with tests that either fail or cannot compile. This pitch is largely based on proposals from this forums thread. Thanks to @CharlesS and @Cyberbeni for the initial direction and t...