lsp: Add unit tests

[jaraonthe-dot-net]

As the groundwork of the language server is done, all the classes (if feasible) in vadl.lsp should be covered with unit tests. This should protect us against regressions going forward.

[flofriday]

In the frontend we have quite a few snapshot tests (FrontendSnapshotTests.java) and we're really happy with how that worked out.
They will have to look differently for the LSP but I cannot recommend the approach enough. 😊