Run karma test on CI against several browsers

[PVince81]

With the karma runner it is possible to run the JS tests against more than just PhantomJS.

The challenge is to have a virtual X server run on CI to which the different browsers can connect to.

To run against Firefox locally for example:

1. make
2. build/node_modules/karma/bin/karma start tests/karma.config.js (or use make test-js-debug after reviewing Makefile improvements core#27222)
3. Open localhost:9876 in Firefox
4. Wait for the tests to run and observe the result on the console

There are settings for karma to automatically start and stop browsers.

@owncloud/qa @DeepDiver1975 @tboerger something for later

[PVince81]

Seems it would be easy on Travis: http://karma-runner.github.io/0.8/plus/Travis-CI.html

Not sure about Jenkins. Might need to fire up a virtual X server.

I wonder if we should just run the JS tests on Travis instead...

[SergioBertolinSG]

What about using sauce labs also for this? https://wiki.saucelabs.com/display/DOCS/Setting+Up+Karma+for+JavaScript+Unit+Testing

In this PR sauce labs is being used: owncloud/core#27191