Change `__check_browser()` to `_check_browser()` (make non-private)

[mdmintz]

Change __check_browser() to _check_browser() (make non-private)

By making this BaseCase method non-private, it'll be easier to override, such as in the case of someone trying to make SeleniumBase more compatible with Appium, while overriding browser_launcher.py via get_new_driver().

[mdmintz]

This was resolved in 4.18.8 - https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.18.8