use assert instead of type cast

Author: yaacovCR

src/execution/__tests__/subscribe-test.ts

@@ -461,13 +461,9 @@ describe('Subscription Initialization Phase', () => {
       'Subscription field must return Async Iterable. Received: "test".',
     );
 
-    (
-      await expectPromise(
-        subscribeWithFn(() =>
-          Promise.resolve('test'),
-        ) as Promise<ExecutionResult>,
-      )
-    ).toRejectWith(
+    const result = subscribeWithFn(() => Promise.resolve('test'));
+    assert(isPromise(result));
+    (await expectPromise(result)).toRejectWith(
       'Subscription field must return Async Iterable. Received: "test".',
     );
   });