grunt: Add Karma task for automated QUnit testing in browsers

grunt: Add Karma task for automated QUnit testing in browsers

To use, ensure npm modules are up to date by running 'npm install'
in tests/frontend. Then run 'grunt qunit' to run it in Chrome.

To run it in both Firefox and Chrome (locally), run

 grunt karma:more

Moved fixture from hardcoded HTML to the testrunner so that our
tests don't depend on arbitrary HTML and thus also pass in browser
contexts where the page only loads QUnit + test suites (e.g. Karma).

Change-Id: I4e96da137340a28789b38940e75d4b6b8bc5d76a