Plugin: Fix test runner. Use babel-register instead of mocha-webpack

[youknowriad]

As noted by @aduth mocha-webpack overrides the default webpack entry, so we'll have issues to make it work with our globals config.

• In this PR, I'm replacing mocha-webpack with babel-register
• Moving the tests close to the source code
• Defining the globals for test in bootstrap-test.js

I avoided publishing directly to #289 PR in case you have objections here.

[aduth]

The downside of this approach is that it doesn't test files as accurately as if they were built from Webpack first (in case we incorporate custom loaders, etc). In my experience this has rarely been problematic, and the upside is that Mocha compilers tend to run much faster than first bundling (since fewer files are touched).

I found that assigning the globals shouldn't be needed, and tests run well without them (removed in 0f1ea2a). If you're seeing otherwise, I was thinking to consider whether this is a result of Webpack's output.libraryTarget (which has a few other options to try: window, global, etc.).

I'm personally fine with this approach but I'll defer to @nylen since he wanted the Webpack integration 👍

[nylen]

I'm personally fine with this approach but I'll defer to @nylen since he wanted the Webpack integration 👍

I just wanted to try it that way because it seemed potentially cleaner than using babel. I'll merge this into #289.