I use a workaround:
Create a new Xcode project, in the same git repo as your-swift-package, without adding any source code files.
Add your-swift-package as a local package to your Xcode project (File-> Add Packages)
Add your-swift-package as a dependency to the Xcode project's target (In the Xcode project select the target and add framework as usual)
Add the test target of your-swift-package to the test scheme of the project (Open the scheme editor in the Xcode project, in test-phase, info tab, add the test target of your-swift-package to the enabled tests using the '+' button
Configure the Xcode project to build in Xcode Cloud
I hope that Apple will soon support building stand alone swift packages, so we do not need this additional workaround.