Added manual test harnesses for React 16.0-16.7

[bvaughn]

Related to issue #1209

Here's a proposal for basic manual test harness to regression test React minor releases for versions 16.0-16.7. (We can add support for older v14 and v15 versions too if we'd like.)

The v16 test app progressively adds React features based on the current minor version so that we can share code between various minor versions.

To use the test harness:

# Cross origin requests are not supported for the file protocol.
# Since the HTML files share a single script file, use http:
pushstate-server .

# View all of the test versions via iframes to test DevTools:
open http://localhost:9000/test/regression/

The test page will look like this:

And DevTools should show all of the versions loaded in their various iframes:

[bvaughn]

Ooh, I think I can make this better using an iframe...