Enhance Test Explorer

rust-analyzer version: 0.3.1877-standalone (574e23ec5 2024-03-09)

I tried current implementation and found many flaws

• always display crates, even there is no tests
  
• display integration test like a crate fixed in #16847
  
• not lazily, which means, it always returns all test cases in the crate when you made changes in any file.
• No output from rustc, this might make user feel something is frozen. I mean something like: fixed in #16845
  

It might not be very easy to fix these things, especially the third point(lazily)

Since we have test explorer anyway now, this channel might be another place to argue about whether we should implement test explorer main part in front-end(the editor) or backend(RA itself).

The client implementation is here: https://github.com/rust-lang/rust-analyzer/pull/14589

Review the client implementation in pull request #14589 and compare the current Test Explorer behavior with the remaining issue, especially eager test-case loading after changes. Confirm which concerns were fixed by #16847 and #16845, then determine whether the remaining lazy-loading and architecture questions belong in the editor client or rust-analyzer backend; done means the remaining flaws have an agreed implementation boundary and verification.