test/unwind: exit with nonzero error code in case of failure

Follow-up to d374d7d23

testsuite/tests/unwind/stack_walker.c

@@ -65,6 +65,7 @@ value ml_perform_stack_walk() {
         printf("TEST FAILED\n");
         /* Re-run the test to produce a trace */
         perform_stack_walk(1);
+        exit(1);
     }
     return Val_unit;
 }