Improvement for window tests for JavaScript-only drivers

During my zombie reanimation project in https://github.com/Behat/MinkZombieDriver/pull/46 I've discovered, that since 2.0.0 version Zombie supports window management and that current window test fails because of:

1. focus isn't switched back to main window, after popup is opened
2. `javascript:` in `href` attribute of links scares Zombie and window content isn't retrieved

I've fixed all of above in this PR.