Support test explorer

[mrcjkb]

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Feature description

rust-analyzer can send notifications that can be useful for producing diagnostics related to :RustLsp testables.

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

For people who don't use neotest to run tests, this may be useful for creating diagnostics with test results.

[mrcjkb]

#778