Add watch-tests and server-test-watcher tasks to Gruntfile to enable liv...

...e reloading of test pages when developing.

I'm not sure about the naming of the tasks, and am happy to change them. I also added both a concurrent task which runs both the server and the watcher and a specific watch task. Unfortunately, the watch task cannot be named 'watch' due to this bug https://github.com/gruntjs/grunt/issues/1260.
Let's change `watch-tests` to call `watch:test` directly. This helps avoid unintended side effects if we were to change `watch-test`.