Test Refactor

[MarkHerhold]

The existing tests are a mess right now and need to be cleaned up.

• fix innefective assertions / swap should.js to chai
• linting
• code coverage
• split tests up
• update test dependencies

I introduced eslint with the airbnb style (it's reasonably similar) for linting and c8 for coverage. In order to keep this PR small and focused, we are not linting/changing the koa-body codebase.

I'm also dropping Node.js 6 from the CI test matrix because it is EOL as of April 2019, which is about when I think we can roll out the next major version of koa-body.

[MarkHerhold]

Pinging @damianb @thuitaw and @zevisert for feedback.

[katanacrimson]

We will need to be sure we're very clear about (effectively) dropping node 6 support here. While yes, it is EOL soon, we are removing it from the tests, which is equivalent to saying we're not checking to see if things don't work on anything under node 8.

Overall though, looks good.

ED: Reread - noticing now that you're implying this will be something for a major release. I like this idea; we should also be considering another issue/pr for introducing full linting of the codebase, and one more for removing the compat shim. I'll open the latter in a few.

[zevisert]

Awesome to see these tests being cleaned up, a few general comments:

1. It would be good to use this time to also bring in something like husky to run tests automatically as a git hook
2. There's still various assertion schemes across the different tests
3. I don't know how you want to enforce my review, so I'm not giving explicitly approving or denying

[MarkHerhold]

@zevisert I switched out should.js for chai. I like your idea to use husky and if you want to PR that, I'd be happy to review.

Thank you both for the review comments.