Using TestRenderer.act() should not silence the ReactDOM warning

[gaearon]

ReactTestRenderer.act() only works for react-test-renderer.
ReactTestUtils.act() only works for react-dom.

Unfortunately due to poor naming, people confuse those two things.

We need to:

• Make sure ReactTestRenderer.act() around react-dom updates doesn't silence the warning, and vice versa. (Currently, it does — but you can fix it by using an empty object that's unique per renderer instead of a true value.)

• Add a special warning message when a different renderer is "acting" updates. That warning message should suggest that you're supposed to use act() corresponding to the renderer you're using, e.g. react-dom/test-utils for react-dom.

This should fix most cases where people wrap updates in the wrong act.

[threepointone]

fixed in #15756, closing this.