chore(cockpit): add client side test infrastructure

Add infrastructure to test the cockpit client application via karma.

To test the cockpit client application proceed as follows:

- install karma (>=  0.9.2) via `npm -g install karma@canary`
- from within cockpit-webapp:
  - run cockpit via `mvn clean tomcat:run -Pdev`
  - start karma via `karma start src/test/js/config/karma.e2e.js`
  - execute single karma test run via `karma run`