Add a task to run the tests once with PhantomJS

The test.js task starts the development server and runs the tests in PhantomJS.  As mentioned in the [readme](https://github.com/openlayers/ol3/commit/b452e04e08fd6b915439f53046a802019a95ccca#diff-e946698b9d76380930fc6477bba3fa68R107), when running the tests continuously during development, it is more convenient to start the dev server (`npm start`) and visit the root of the test directory in your browser.

Later we can bring in Karma to drive PhantomJS and other browsers, but this simple "run once" task is useful for the CI job.