Fix child process killing when the parent process SIGINTs.
If you `ctrl + c` the Puppeteer parent process, we would sometimes not properly handle killing of the child processes. This would then leave child processes behind, with running Chromium instances. This in turn could block Puppeteer from launching again and results in
cryptic errors.
Instead of using the generic `process.kill` with the process id (which for some reason is negative the pid, which I don't get), we can kill the child process directly by calling `proc.kill`.
Fixes#5729.
Fixes#4796.
Fixes#4963.
Fixes#4333.
Fixes#1825.
We have some bug where some test runs will pass but then stall as Mocha
doesn't exit cleanly. This is proving very hard to track down (I've yet
to replicate it or find the test that causes it) and it doesn't happen
consistently.
The `exit` flag forces Mocha to hard exit. This will help with CI
stability. The flag only gets set on CI runs.
This commit updates all the non-Puppeteer unit tests to run using Mocha and then deletes the custom test runner framework from this repository. The documentation has also been updated.
