Switch to tape for testing

I have strong feeling, that we have poor tests:

 * We test got against real hosts - it takes too much time
 * All tests are stored in one file
 * Not all cases are covered
 * ~~Mocha is quite bloated, when `before` and `after` used~~

This PR porting parts of request tests (and ssl certificates). 

For now `test-requests.js` is hanging on redirects, when `agent` is not `false`.
https://github.com/sindresorhus/object-assign