Barebones LSP for integration testing

[kirawi]

I think it would be a good idea to test our LSP code by having a barebones LSP with some helper functions. Something like this could have caught #9429 if we had a test where we have some diagnostics, e.g. helper_lsp.set_diagnostics(&[(0, 0, "diagnostic here")]) and tested if it appeared in the document itself.

[Omar-Elrefaei]

Is this still relevant in light of this comment? #6486 (comment)

I'm very wary of adding integration tests against a language server. ....... On balance I don't think LSP integration tests are worth their added weight and I would prefer we test LSP features manually.

If not, I might be interested in taking this on.