Ruby: use Testing view to discover and run unit tests

[ntrogh]

The Ruby LSP extension supports test discovery and running with the Testing view in VS Code (https://shopify.github.io/ruby-lsp/#test-explorer).

• [ ] Update the Ruby language doc to include the testing scenario. Provide a concrete example and refer back the Ruby LSP docs for more details.
• [ ] Update Testing user guide to reference Ruby testing in next steps