Adopt rstest for parameterized Rust tests #72

Problem

Rust has no built-in way to write one test for many inputs. Tests that check a list of inputs do it in one of two ways:

• They loop over an array inside one #[test].
• They put several asserts in one function, often with comments that say why each input is there.

Both ways have the same problems:

• The test stops at the first input that fails. The inputs after it do not run, so one run does not show all the failures.
• The test reports one failure for the whole function. The output names the input only if the assert message prints it.
• cargo test cannot run one input by itself.

Proposed solution

Adopt rstest as the tool for parameterized tests. Each #[case] is a separate test: it passes or fails by itself, its name shows in the output, and cargo test can filter by that name.

rstest is the most used parameterized-test crate for Rust. On crates.io it has about 2.4 times the recent downloads of test-case, the next most used crate, and about 3 times as many dependent crates. It has a release every few months, and it works with #[tokio::test] async fn.

Before:

#[test]
fn test_rejects_unsupported_schemes() {
   for url in ["ftp://example.com/a", "file:///tmp/a", "ws://example.com/a"] {
      assert!(validate_url(url).is_err());
   }
}

After:

#[rstest]
#[case::ftp("ftp://example.com/a")]
#[case::file("file:///tmp/a")]
#[case::websocket("ws://example.com/a")]
fn test_rejects_unsupported_schemes(#[case] url: &str) {
   assert!(validate_url(url).is_err());
}

A failure is then reported as test_rejects_unsupported_schemes::case_2_file.

When to use rstest

Use #[rstest] with #[case] when a test runs the same check on a list of inputs. Name each case (#[case::name(...)]), so the name says what the input is. A name can replace a comment that explained the input.

Keep a plain #[test] when:

• The test is a scenario with its own setup and asserts.
• The test loops over results from one shared setup. The loop is then part of the scenario, not a list of inputs.

Out of scope

• rstest fixtures (#[fixture]) for shared setup. These can be a later issue.
• #[values] matrices and #[files] cases. Use these only when a test needs them.

Acceptance criteria

• rstest is a dev-dependency of each crate that has parameterized tests.
• Tests that loop over a list of inputs, or that put several inputs in one function, use #[rstest] with one named case for each input.
• No input is removed. The number of inputs checked is the same as before.
• cargo test --workspace passes, and cargo test can run a single case by its name.