Improve reliability of xthread cancellation test.

c++/src/kj/async-xthread-test.c++

@@ -338,16 +338,17 @@ KJ_TEST("cancel cross-thread event while it runs") {
       exec = &KJ_ASSERT_NONNULL(*lock);
     }
 
-    volatile bool called = false;
     {
+      volatile bool called = false;
       Promise<uint> promise = exec->executeAsync([&]() -> kj::Promise<uint> {
         called = true;
         return kj::NEVER_DONE;
       });
+      while (!called) {
         delay();
+      }
       KJ_EXPECT(!promise.poll(waitScope));
     }
-    KJ_EXPECT(called);
 
     exec->executeSync([&]() { fulfiller->fulfill(); });