Use experimental/discoverTests to find tests

[mrcjkb]

Checked other resources

• This is not a bug report or a question (Please use issues for those).
• I have searched existing discussions.
• I have searched existing issues.
• This cannot be implemented in rust-analyzer.
• I have added a very descriptive title to this feature request.

Feature description

Currently, rustaceanvim uses experimental/runnables to find targets and filters out non test targets.
experimental/discoverTests might be more reliable/simpler on the client side.

https://rust-analyzer.github.io/book/contributing/lsp-extensions.html#test-explorer