XCTest app-hosted tests can't run without showing a window

If a macOS app's unit tests use the app itself as the test host, testmanagerd launches it as a normal foreground GUI app. Under test, the app's own startup code (setActivationPolicy(.regular), SwiftUI WindowGroup) still runs, so it gets a Dock icon and its main window shows. xcodebuild has no equivalent of open -g -j to start the host in the background. Making the tests' own windows borderless and off-screen doesn't help, because the host app is what shows. To test without disturbing a user at the machine, either add a DEBUG-only env flag that sets .prohibited and skips the main window under XCTest, move the tests to a hostless logic-test bundle or SwiftPM package, or schedule host-based runs for when the machine is idle. build-for-testing is safe at any time because it launches nothing.