[io/tests] gracefully terminate the runtime on success

This allows the examination of stats which get printed in the Runtime destructor. Also remove useless stdlib includes (Why does iwyu not catch those?).

tests/io/ConcurrentNetworkEchoTest.cpp

@@ -54,12 +54,12 @@ auto main(int argc, char* argv[]) -> int {
 					cps);
 		}
 		cps.wait();
-		exit(EXIT_SUCCESS);
+		runtime.initiateTermination();
 	});
 
 	runtime.scheduleFromAnywhere(*client_producer);
 
 	runtime.waitUntilFinished();
 
-	return EXIT_FAILURE;
+	return EXIT_SUCCESS;
 }