The purpose of this PR is to be able to easily run karma tests in other
browsers without manually editing the karma configuration or making
local copies of the karma configuration.

Additions:

- `yarn test --no-browser` to avoid launching the default browser.
  It is up to the developer to run the tests in whatever browser she/he
  chooses by navigating to http://localhost:9876/.

- `yarn test --browser <browser>` run the tests in a different
  browser/s instead of the default browser. Chrome launcher comes by
  default with karma. Firefox, Safari and others can be installed
  independently.

Modifications:

- `yarn gulp test-watch` has been substituted by `yarn test --watch` for consistency.