Fix multithreading with multi proxy

[phongdatgl]

If you use multithreading with multi proxy, it will use only one proxy created at downloaded_files/proxy_ext_dir folder. I make random folder name to fix it.

[mdmintz]

This PR is no longer necessary due to https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.13.23
The new --multi-proxy pytest option, (multi_proxy=True from methods), will now create a uniquely-named proxy/auth config. That way, if running many tests in parallel that all use the same proxy, there won't be duplicate proxy config files taking up a lot of unnecessary space. Also, instead of a random number, it will increment until an available number is found. This makes it easier to keep track of files. And note that os.remove() is for files, (not directories).