Fix type in test

Summary: fail isn't a thing

Reviewed By: passy

Differential Revision: D16335797

fbshipit-source-id: c2f07b722424f540e6eb1a4cd423397ec1fe545c

src/utils/__tests__/Idler.node.js

@@ -17,8 +17,7 @@ test('Idler should interrupt', async () => {
       }
       await idler.idle();
     }
-    // $FlowFixMe T47375728
-    fail('Idler should have thrown an error');
+    expect('error').toBe('thrown');
   } catch (e) {
     expect(i).toEqual(100);
   }