let the simple cpyext demo work on sandboxed

Author: timfel

graalpython/com.oracle.graal.python.test/src/tests/cpyext/demo.c

@@ -49,7 +49,11 @@ static PyObject* demo_system(PyObject *self, PyObject *args)
 
     if (!PyArg_ParseTuple(args, "s", &command))
         return NULL;
-    sts = system(command);
+    if (!strcmp("echo 1", command)) {
+        sts = 0;
+    } else {
+        sts = 1;
+    }
     return Py_BuildValue("i", sts);
 }