Things that are required:
- Browser that is fast
- GUI not required
- Runs of Windows/Linux
- Big plus if free
Take a look at PhantomJS.
PhantomJs is a "headless browser", which will let you concentrate on your JS without having to worry about a GUI. It is widely used for testing on many FOSS projects.
HtmlUnit is a "GUI-Less browser for Java programs". It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc... just like you do in your "normal" browser.
You can use it directly, or use other tools which are based on it. The homepage has links to these.