Forking process should throw error

Since multiprocess is not allowed in competitive programming, it should throw an error if submit the following code.
```c
#include <unistd.h>
using namespace std;
int main() {
    while (true) fork();
    return 0;
}
```
* If we use "isolate", it will keep running and get a T instead of an X.
* If we doesn't use it, the grader process will be down because it doesn't handle the case.