Babelify only test files

[vadimdemedes]

I think we should apply babel only to test files, e.g test.js, test/test-something.js.
Currently, if you have this common code:

const test = require('ava');

const mylib = require('./');

mylib will also be run through babel and that is not good, because that way AVA transforms library's source code. This behavior could be fixed by setting a regex for require('babel/register') to whitelist only test files.

[sindresorhus]

👍 We already know which files are the test files, so we can just blacklist anything that is not passed into AVA.