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