Add RUST_BACKTRACE: full to lsp CI tests

[JoshuaBatty]

Description

This will help when trying to debug why LSP tests are failing everynow and then in CI, such as this run.

closes #6456

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
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• I have added tests that prove my fix is effective or that my feature works.
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