Discover tests using a custom test harness

First, thank you for your hard work on rust-analyzer.

We consider migrating from dirtest to datatest-stable. dir-test exports a macro that generates a #[test] function for every file in a given directory. This is great because it plays really nicely with r-a: r-a detects the tests and we can run them by name. However, it has the downside that it requires a build.rs file that invalidates the macro whenever one of our spec files change, which leads to very slow iteration cycles.

That's why we consider migrating to datatest-stable. It uses a custom test harness that mimics cargo test. This is great because it removes the need for the build.rs file and the need to recompile the tests after making changes to the spec files. However, we lose the r-a integration because the tests are now no longer expanded in a macro.

Supporting this use case is tricky because r-a can't make any assumptions about a custom test harness. One possible solution would be to support fetching the known tests using cargo test -- --list (ideally with a custom filter). Obviously, it would fail to automatically detect new tests but that's already the case with our existing dirtest setup where it's necessary to manually clear the macro-cache.

Start by tracing rust-analyzer's existing test discovery and integration with the dir-test macro. Then investigate how a custom harness exposes tests through cargo test -- --list, including whether a custom filter is possible. Done means custom-harness tests can be discovered and run by name without relying on macro expansion.