How to run Selenium system tests without requiring to open the browser?
No. Selenium is written in JavaScript; it's designed to run in a real browser to test compatibility with a real browser. There are a variety of other tools out there designed to run tests that simulate a browser; you can look into HtmlUnit or Canoo WebTest.
Hope this will help you.
Have you tried XLT?It doesnt require an open browser at allhttp://www.xceptance.com/products/xlt/what-is-xlt.html
You can run all your tests in one browser instance if you like. You just have to pass your webdriver instance to each test. Like having a singelton WebDriver in a static class from where all your testcases can access the WebDriver. Works fine and is usefull if you got a session to keep