// this assures that cancelling threads made in the same realm works.
// for cancelling across realms, another test would be needed.
svoid testThreadCancellation() {
  new Flag flag;
  Thread t = startThread("testThreadCancellation", r {
    try {
      busySleepSeconds(20.0);
    } finally {
      flag.raise();
    }
  });
  sleepSeconds(1.0);
  assertFalse(flag.isUp());
  cancelThread(t);
  assertTrue(waitUntil(50, 2.0, () -> flag.isUp()));
  print("Local thread cancellation works");
}