how to refresh the browser in [Before] for each test running?

I doubt this is the behavior you are looking for. If you refresh the browser, FlexUnit itself will be closed and relaunched along with the Flash Player plugin. Your test will never be run, your browser will just refresh and reload the SWF.

That said, I think you can do this by calling into javascript. This example is untested, but hopefully it will put you on the right track:

getURL("javascript:window.location.reload(true)");