Fix invalid interface conversion in runner

panic: interface conversion: interface {} is syscall.WaitStatus, not unix.WaitStatus

goroutine 1 [running]:
main.runTestCaseNative(0xc0001fc000, 0xe3, 0xc000119b60, 0x1, 0x1, 0x0, 0x0)
	test/runner/runner.go:185 +0xa94
main.main()
	test/runner/runner.go:118 +0x745

PiperOrigin-RevId: 361957796

runsc/specutils/namespace.go

@@ -275,7 +275,7 @@ func MaybeRunAsRoot() error {
 	}()
 	if err := cmd.Wait(); err != nil {
 		if exit, ok := err.(*exec.ExitError); ok {
-			if ws, ok := exit.Sys().(unix.WaitStatus); ok {
+			if ws, ok := exit.Sys().(syscall.WaitStatus); ok {
 				os.Exit(ws.ExitStatus())
 			}
 			log.Warningf("No wait status provided, exiting with -1: %v", err)

test/runner/runner.go

@@ -182,7 +182,7 @@ func runTestCaseNative(testBin string, args []string) error {
 	}
 
 	if err := cmd.Run(); err != nil {
-		ws := err.(*exec.ExitError).Sys().(unix.WaitStatus)
+		ws := err.(*exec.ExitError).Sys().(syscall.WaitStatus)
 		return fmt.Errorf("test exited with status %d, want 0", ws.ExitStatus())
 	}
 	return nil