Move to AVA for testing

Let’s move all testing related things into a test directory. The current Mocha test groups should just be separate test files.

[sindresorhus]

@tdeschryver Some other project that needs to be converted from Mocha to AVA if you’re interested:

• https://github.com/yeoman/stringify-object
• https://github.com/yeoman/update-notifier
• https://github.com/yeoman/insight
• https://github.com/sindresorhus/string-length

[timdeschryver]

@sindresorhus I’ll try to send a PR a day 😃