Change a test to be relevant in Fiber (#8281)

Fiber supports fragments so we will use undefined instead.
This test is then valid both in Stack and in Fiber.

src/renderers/dom/__tests__/ReactDOMProduction-test.js

@@ -178,7 +178,7 @@ describe('ReactDOMProduction', () => {
     expect(function() {
       class Component extends React.Component {
         render() {
-          return ['this is wrong'];
+          return undefined;
         }
       }