Stop document.createElement in tests

[zpao]

We have a ton of places that do

var container = document.createElement('div');
var component = <Something />;
var instance = React.renderComponent(component, container);

We should just be doing

var component = <Something />;
var instance = ReactTestUtils.renderIntoDocument(component);

This isn't an exciting task, but gets you exposed to lots of tests which can help with understanding react core better. cc @kmeht

[zpao]

@sebmarkbage has a codmod coming that touches a bunch of these, so don't jump in quite yet.

[kmeht]

Okay, keep me posted. I'll manually fix any callsites that slip through the codemod.

[zpao]

Codemod in. He wasn't actually fixing them, just touching code around them so would have been merge hell.

[danielschonfeld]

Is it safe to try to get those patched now?

[zpao]

It should be. Feel free to take it on. Let me know if you have any questions!