Prevent bundling of Node polyfills when importing TestUtils/TestRenderer

[gaearon]

I think this fixes #14853 (comment).

In particular, there are two cases we'd like to avoid:

• We don't want Webpack to emit a warning about require expression being dynamic.
• We don't want Webpack to include its Node polyfills (for example, for timers or for global) because we access them. (This is happening on master.)

Effectively, we want to obscure the require call from webpack. There are a few webpack-specific ways to do it but they seem brittle. Additionally, naïve ways to obscure it (like the current code) are inlined by GCC at our build step for the prod bundle.

Instead, I'm reading module.require — which doesn't even exist in webpack runtime but exists in Node. Webpack doesn't analyze it. To further strengthen it, I make the require string itself dynamic. Seems like this is sufficient. I only tested webpack and Node. It might be worth testing that browserify or Rollup work as expected — but they also not known for noisy warnings so I assume it should be fine.

This wouldn't work if TestRender or TestUtils were a Node ESM because there would be no require in it. But that's status quo anyway because our current require call wouldn't work either.