Latching onto the “library” keyword, and assuming a transient Xcode project file is allowed, that has already been possible for quite some time:
swift package generate-xcodeproj
xcodebuild build -sdk iphoneos -scheme 'MyPackage-Package'
xcodebuild test -destination 'name=iPhone 8' -scheme 'MyPackage-Package'
(but replace “MyPackage” with the actual package name declared in the manifest.)