build: Use headless Chrome and Firefox for QUnit testing

build: Use headless Chrome and Firefox for QUnit testing

Also:

* Remove "watch" task from Gruntfile.
  This wasn't exposed via npm-run and afaik rarely if ever used.
  It seems mostly obsoleted by modern editors which perform
  linting and style guidance as-you-type and on-save.

* Remove "chromium" target from karma config, the default
  target supports both Chromium and Chrome.

* Remove unused "default" alias in Gruntfile. Was only
  accessible if developers installed grunt-cli globally, which
  we don't require and shouldn't encourage. The new minify task
  is now exposed as "npm run build".

* Remove "test" alias in Gruntfile. Was only used by "npm run test",
  which now uses "grunt lint" directly, which makes it more clear
  that these are only linters, and not unit tests.

This references T211784 because ensuring no headed browsers are used
(which would need Xvfb to start), is a prerequisite for the new Node 10
images. We may still need to provide a slower Xvfb-wrapped variant for
some repos, but would be nice of mw-core wasn't one of them.

Bug: T211784
Change-Id: I270a16d2b4f799ac7c9f84e9e2ee23806bdbc57e