When Apple showed Swift Playgrounds 4 with the possibility of developing and publishing apps entirely on the iPad I think I'm not gonna make too huge a leap by saying that some people were delighted with the opportunity,
One thing missing there is the possibility to write tests of any kind.
PlaygroundTester is an SPM package that bridges that gap - allowing you to add tests to your Swift Playgrounds 4 app project, with automatic discovery and execution!
It doesn't yet have parity with XCTest but I hope to bring it there sometime.