in Sources/XCTest/Public/Asynchronous/XCTWaiter.swift [370:377]
func cancelPrimitiveWait() {
guard let runLoop = runLoop else { return }
#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
CFRunLoopStop(runLoop.getCFRunLoop())
#else
runLoop._stop()
#endif
}