[tests] Add tests for socks proxies

[coletdjnz]

This PR implements some long needed test cases for our socks proxy module. Previously, only a few e2e tests were implemented and were disabled by default.

As part of this I have written basic socks4/5 dummy proxy servers (as per the specs) that allow us to get back proxy debug information to test against.

Instead of testing the socks.py module directly, I am testing at the handler level. This is to allow us to test socks proxy implementations in multiple handlers. Additionally, the tests are written to work with different protocols other than http (for when we need to implement websockets over socks, for instance).

When writing these tests, the following bugs/issues with our socks client implementation have been found:

• IPv6 socks4/5 proxies are not supported (SOCKS proxy mechanism does not support IPv6 ytdl-org/youtube-dl#15368)
• IPv6 over socks5 is not supported
• --source-address is not obeyed for either socks4 and socks5
• Using an IP instead of a domain name for a destination does not work with socks4a due to a bug in the implementation

Tests have been written for these cases but are currently skipped until they are fixed. I plan to look into these in followup PRs.

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Copilot Summary

🤖 Generated by Copilot at 37438eb

Summary

📄🔧🧪

Refactor networking tests and add a handler fixture. This change improves the code structure and reusability of the tests for different networking backends.

Pytest fixture of doom, testing the handler of the beast
Refactoring the imports, breaking the chains of the code
Conftest.py is the file, where the magic happens
Networking backends of fire, burning the bridges of the web

Walkthrough

• Add a new pytest fixture handler in conftest.py that returns a partial function of a request handler class based on the parameter passed to the test function (link)
• Use the handler fixture to test different networking backends for yt-dlp, such as urllib, pycurl, etc. (link, link, link)
• Set a fake logger for the request handler to avoid printing messages to the console during testing (link)
• Move some of the imports from test_networking.py to conftest.py since they are only used by the handler fixture and not by the test functions (link)
• Remove the import of _REQUEST_HANDLERS from test_networking.py since it is no longer needed there (link)
• Remove the definition of the handler fixture from test_networking.py since it is now moved to conftest.py (link)

[Grub4K]

Cosmetic things mainly