test form elements in isolation

Hello,

I have an input on which I use Decorator, it works well in browser, however when I try to test it without wrapping it in Form element I get `Cannot read property 'attachToForm' of undefined`
Is there a way to test form elements in isolation without having to wrap them in Form tag?

The issue names Decorator and Form elements but no files or tests. First reproduce the `attachToForm` error with an input rendered without a Form, then trace the isolation behavior through the relevant component entry points. Done means the isolated form element can be tested without the error, with coverage for both isolated and Form-wrapped usage.

Written by the indexing model from the issue text.