tests: select available TCP port

[mbeijen]

Summary

If TCP port 8000 is in use on the developers' machine, the tests hang. It is mentioned in the CONTRIBUTING.md but it is not great developer experience.

Let's instead select a random free port by specifying port=0 in uvicorn.

Fixes #681

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing mbeijen:tests-should-not-depend-on-port-8000 (e11dcfe) with main (fee387a)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Kludex]

Can you try what I commented, please? If it doesn't work, this is fine.

[Kludex]

I think we can avoid port=0 and the introspection of servers object in Uvicorn if we use free_tcp_port, and then we can use self.config.port.

Suggested change

	config = Config(app=app, lifespan="off", loop="asyncio", port=0)
	config = Config(app=app, lifespan="off", loop="asyncio", port=free_tcp_port)

[mbeijen]

Can you try what I commented, please? If it doesn't work, this is fine.

I was able to make it work but only using free_tcp_port_factory https://anyio.readthedocs.io/en/stable/testing.html#built-in-utility-fixtures because that one is session-scoped. I updated my PR

[Kludex]

Amazing. Thanks! 🙏