Replace most external test APIs with local mock server

[jtdowney]

This adds a mock HTTP server that operates similarly to https://httpbin.org. There are two places where it still uses the old tests, but there is a line of sight to moving those over as well.

1. invalid_tls_test expects a "typical" TLS error like an expired certificate. I felt like it would change the spirit of the test to have it test against an unknown CA. This can move to a local test PKI once Add custom cacert support #36 is finished.
2. ipv6_test could be replaced with a request to http://[::1]:8080 once IPv6 addresses with brackets are not handled correctly.  #34 is finished.

With a local test PKI, additional testing could be added for #55 as well.

Closes #56

[lpil]

ipv6_test could be replaced with a request to http://[::1]:8080 once IPv6 addresses with brackets are not handled correctly. #34 is finished.

I tried to add this but I got this failure. Do you know what this might be?

let assert test/gleam_httpc_test.gleam:106
 test: gleam_httpc_test.ipv6_test
 code: let assert Ok(resp) = httpc.send(req)
value: Error(FailedToConnect(Posix("eafnosupport"), Posix("econnrefused")))
 info: Pattern match failed, no pattern matched the value.

[lpil]

Oh I'm a dummy, I used the wrong port, didn't I?

[jtdowney]

Took me a few minutes to remember what I was doing here. Now that #64 is in, ipv6_literal_host_test is not needed in this branch because ipv6_test is hitting an IPv6 bracketed literal just as it was. It just wasn't cleaned up because this branch predates it.

The only thing we lose, as far as I can tell, is a test that shows a domain name resolves to IPv6 correctly, but that is sort of testing httpc a bit. We have a test that shows httpc can talk IPv6 to a local server.

[lpil]

Thank you!!!!