Adopt VSCode's native testing/test explorer API

[fwcd]

Motivation

VSCode provides a new testing API with version 1.59:

• https://code.visualstudio.com/api/extension-guides/testing
• https://jpearson.blog/2021/09/01/test-explorer-in-visual-studio-code/

Description

It would be cool if we could integrate this, e.g. in a similar way to #92, perhaps using a new non-standard LSP request for resolving test cases and then running them through the debug adapter.