Proposal: use a small test server for testing rather than external services

[jtdowney]

I was starting to play with extending gleam_httpc for something, and ran into the tests failing due to an external service change. From my experience and a brief survey of the Go, Python, and Rust ecosystems, I was surprised to see gleam_httpc only tests against external services instead of starting a small test server with known responses and hitting that. This becomes more relevant the more feature-rich you need the tests to be (mTLS, custom CAs, streaming, etc.) or when you need to test edge cases. I am sure this was considered before, but I didn't see any prior issue discussing the approach.

[lpil]

Sounds good to me! I agree it will be especially useful for more complex tests in future.