Could you please provide a bit more information about your project configuration? Best would be a self containing SwiftPM Package e.g. hosted on GitHub.
Complete code sample and instructions how you compile the code would also work :)
Yeah I suspect it's because on macOS the Objective-C XCTest runtime is present on the system and implicitly available to Swift, but not intended to be used with the REPL. On Linux it's not there so there's nothing to reference until link time when compiling tests. I guess the question is what do you expect to see?