test: port tests from node-http-proxy

[pi0]

Add tests from upstream to cover ~85% of the current code.

Some tests were marked as todo (mainly socket.io and error-related ones) as I didn't have time to investigate more.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (81f5e57) to head (8c61642).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #88       +/-   ##
===========================================
+ Coverage   43.78%   86.43%   +42.64%     
===========================================
  Files           8        7        -1     
  Lines         660      634       -26     
  Branches       79      177       +98     
===========================================
+ Hits          289      548      +259     
+ Misses        369       86      -283     
+ Partials        2        0        -2     

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