securedrop.tests.functional.web_drivers: refactor to parameterize on driver class

[cfm]

As I found out in https://gist.github.com/cfm/8a4dddcee3e993998ec4d68e123da89d, it would be trivial to refactor securedrop.tests.functional.web_drivers into a single _create_driver(driver_cls, **kwargs) function. tbselenium.utils.set_tbb_pref() works isomorphically on selenium.webdriver.firefox.webdriver.WebDriver as on tbselenium.tbdriver.TorBrowserDriver.

[alberefe]

Hi, I'd like to help but I'm not sure if I'm understanding the issue. It is to create a unified function for creating the driver, firefox or tor, based on the class passed as argument, am I correct?