Add integration tests

[novemberborn]

Most Babel related tests have been removed from AVA. We should write integration tests here instead.

Hopefully we can install AVA from GitHub (avajs/ava#master) in a subdirectory, and then write tests that call ava and observe the output. Much like we did in AVA itself (except there the tests were hosted in tap). See avajs/ava#2319 for the tests that were removed.

[sindresorhus]

I think you could add AVA as a dev dependency using an alias: https://npm.community/t/documentation-for-package-aliases/10039