Do not wait for events to be dispatched to IDBRequest

The test expects event to be dispatched to IDBRequest after its execution
context is stopped, but this is not standard behavior. WebKit and Blink can stop
dispatching event as soon as iframe is detached (iframe.remove() is invoked),
which leads to test timeout. Since the goal of the test is to verify the accessor
is still valid after execution context is destroyed (no exception is thrown), we
can just check readyState without wait. We only check the type instead of value
as different engines handle the destruction differently, so the state may not be
updated right away.

IndexedDB/ready-state-destroyed-execution-context.html

@@ -21,11 +21,7 @@
     const openRequest = iframe.contentWindow.indexedDB.open(dbname);
     assert_equals(openRequest.readyState, 'pending');
     iframe.remove();
-    await new Promise(resolve => {
-      openRequest.onerror = resolve;
-      openRequest.onsuccess = resolve;
-    });
-    assert_equals(openRequest.readyState, 'done');
+    assert_equals(typeof openRequest.readyState, 'string');
 }, 'readyState accessor is valid after execution context is destroyed');
 
 </script>