This patch aligns Puppeteer testing infrastructure with the approach
we use in Puppeteer-Firefox.
This patch:
- makes all tests accept Puppeteer object as a function argument
rather than require it statically. This way we can pass either
Puppeteer or Puppeteer-Firefox to drive tests.
- renames the `puppeteer.spec.js` into `launcher.spec.js`. The
`puppeteer.spec.js` is now the entry point for all cross-browsers
tests.
This patch adds a new require, `puppeteer/Errors`, that
holds all the Puppeteer-specific error classes.
Currently, the only custom error class we use is `TimeoutError`. We'll
expand in future with `CrashError` and some others.
Fixes#1694.
