Tests on iOS Simulator are limited to a single thread

When running tests with XCTests/Xcode with the iOS Simulator I can print out Thread.current and see it only allows the main thread and a single background thread to be used. With macOS it allows more threads. This is causing tests to run with serial behavior which is different from the normal runtime. It makes testing with the iOS Simulator unreliable. And running tests with macOS will require code signing which can be challenging for CI/CD.

There is this comment on the Async Algos package on GitHub which indicates testing with the iOS Simulator uses single thread and I do not see a way to change that. Can anyone explain why it is this way and perhaps a setting I can use to enable multiple threads?