ntoskrnl.exe/tests: Always test DeviceIoControl result.

dlls/ntoskrnl.exe/tests/ntoskrnl.c

@@ -233,6 +233,7 @@ static void test_overlapped(void)
 
     cancel_cnt = 0xdeadbeef;
     res = DeviceIoControl(file, IOCTL_WINETEST_GET_CANCEL_COUNT, NULL, 0, &cancel_cnt, sizeof(cancel_cnt), NULL, &overlapped);
+    ok(res, "DeviceIoControl failed: %u\n", GetLastError());
     todo_wine
     ok(cancel_cnt == 2, "cancel_cnt = %u\n", cancel_cnt);