Rather than maintain our own test runner we should instead lean on the community and use Mocha which is very popular and also our test runner of choice in DevTools too.
Note that this commit doesn't remove the TestRunner source as it's still used for other unit tests, but they will be updated in a future PR and then we can remove the TestRunner.
The main bulk of this PR is updating the tests as the old TestRunner passed in contextual data via the `it` function callback whereas Mocha does not, so we introduce some helpers for the tests to make it easier.
This patch introduces a page.waitForFileChooser() method
that adds a watchdog to wait for file chooser dialogs.
This lets Puppeteer users to capture file chooser requests
and fulfill/cancel them if necessary.
Fixes#2946
Certain Puppeteer methods do expose the inner browser - e.g.
`browser.version()` depends on the browser we run.
Split out these tests into a vendor-specific test suites.
References #3889
