Fix issues with ruby-sample integration test

[edmorley]

The test has several issues:

1. The test uses call_test_fixture_service rather than something more realistic like ureq
2. The web server isn't compliant with the HTTP spec (so attempts to use ureq to make a request to it fail)
3. The app is unnecessarily complicated - it does not need to accept a posted value and invert it - replying with a fixed string to a GET would do (eg just using a Ruby stdlib HTTP server)
4. It uses a hardcoded sleep, rather than something like:

   libcnb.rs/libcnb-test/tests/integration_test.rs

   Lines 77 to 87 in 2a75bb6

   	// Retries needed since the server takes a moment to start up.
   	let mut attempts_remaining = 5;
   	let response = loop {
   	let response = ureq::get(&url).call();
   	if response.is_ok() || attempts_remaining == 0 {
   	break response;
   	}
   	attempts_remaining -= 1;
   	thread::sleep(Duration::from_secs(1));
   	}
   	.unwrap();

5. It uses Ruby 2.7, so has to use the heroku/buildpacks:20 builder image (rather than the heroku/builder:22 used by other tests), which slows down CI as it means another builder image has to be pulled.

[schneems]

The web server isn't compliant with the HTTP spec (so attempts to use ureq to make a request to it fail)

Can you give me some more context here? What fails?

[edmorley]

@schneems The test app is not a proper HTTP server, since it uses TCPServer and does not implement the HTTP protocol itself. It echos back the received payload reversed only, and is missing the status line, headers etc:
https://github.com/heroku/libcnb.rs/blob/main/examples/ruby-sample/test-fixtures/simple-ruby-app/app.rb

[edmorley]

Wontfix since the Ruby example has been removed in #757.