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.
Okay. I'm going to change it to `google-chrome` since that *should* open up the correct browser. If it doesn't work, we shouldn't have to worry too much: the user can still open up the URL manually.