improve proxy unit tests

[mga-chka]

Description

This PR fix and improve unit test on proxy.go

Pull request type

Please check the type of change your PR introduces:

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

• [x] Linter passes correctly
• [ ] Add tests which fail without the change (if possible)
• [x] All tests passing
• [ ] Extended the README / documentation, if necessary

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Further comments

[render[bot]]

Your Render PR Server URL is https://chproxy-pr-203.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cbqf7h02i3mj3e1k40bg.

[mga-chka]

FYI I've run the ci on github actions a dozen of times and it failed only once on a unit test that is not linked to my modification (TestServe/http_cached_gzipped_deadline). The most instable test so far (TestReverseProxy_ServeHTTP1/queue_overflow_for_user) seems stable

[gontarzpawel]

For future improvement to avoid this kind of complicated issues we should have a fake server handler scoped to each test case. Tests will be a bit longer, but I doubt it will be very impactful