feat: render failing #[test] results as source diagnostics

[ivov]

Adds failure reporting for #[test] functions. A test may now return Result<(), error> and end in Err. When this happns, lis test renders the failure as a diagnostic that underlines the test and shows the returned error.

import "go:errors"

#[test]
fn parses_header() -> Result<(), error> {
    Err(errors.New("missing Content-Type"))
}

lis test reports:

  [error] test returned Err
   ╭─[http.test.lis:4:4]
 3 │ #[test]
 4 │ fn parses_header() -> Result<(), error> {
   ·    ──────┬──────
   ·          ╰── missing Content-Type
 5 │     Err(errors.New("missing Content-Type"))
   ╰────

Important

The test runner is work in progress, not ready for use yet!

Follow-up to #783