Separate createElement and JSX tests

This essentially copies all classic element creation tests to the modern
JSX tests. The classic tests doesn't use JSX and modern tests do.

The idea is that the JSX tests can start dropping dynamic checks once
we have Flow support for those features. JSX won't be necessary for
dropping dynamic checks. Plain object will also work. Flow will also not
be necessary for JSX. However, the tests should test for the suggested
combination (JSX + Flow).

This also moves some misplaced tests to ReactDOM and Validator.

Note that the modern tests uses ES6 classes. I will add separate tests for
those. However, these are not guaranteed to have .displayName so all our
error messages should check .name if available instead. This should be
better abstracted but I just adhoc fix this for now.
Merge pull request #2748 from sebmarkbage/modern

Separate createElement and JSX tests