Use puppeteer for testing

[jankapunkt]

We need to run tests with puppeteer to simulate browser env, too.

Also the test "supports long stack traces" contains a muted line, that depends on window.navigator, which requires puppeteer to have this variable present.

[jankapunkt]

This turns out to be much harder than expected. It's pretty easy to install and launch puppeteer but executing the tests within the browser js context reqires to spawn the full mocha suite within this env, otherwise the parts executed in the browser env will miss all necessary scope from the test suite. Will investigate how meteortesting:mocha does it, since they got it running, too this way