Remove /testing entry point from 'react' package (#18137)

We're not actually building this entry point. I can't think of a reason
we'd need to fork the isomorphic one. We don't really fork it for
anything since it's so generic to work with all renderers.

Since /profiling doesn't have this, it might confuse the story if we made
people alias two things for testing but not profiling.

Author: sebmarkbage