Remove all usages of return value for ReactDOM.render and TestUtils.renderIntoDocument

[azasypkin]

As per [1] we shouldn't rely on return value of ReactDOM.render and TestUtils.renderIntoDocument because of:

ReactDOM.render() currently returns a reference to the root ReactComponent instance.
However, using this return value is legacy and should be avoided because future versions
of React may render components asynchronously in some cases. If you need a reference
to the root ReactComponent instance, the preferred solution is to attach a callback ref to
the root element.

It's also a reason of some unit tests intermittent failures we have currently.

[1] https://facebook.github.io/react/docs/top-level-api.html