Use cargo-nextest with retries for flaky LSP test

[JoshuaBatty]

Description

This PR introduces cargo-nextest to improve our test runs. We've configured 2 retries for the go_to_definition_for_paths test, which has been intermittently failing. The LSP CI workflow has been updated to use cargo-nextest.

Should mitigate #6029 until we get to the root cause why these tests are flaky.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6521 will degrade performances by 17.77%

_{Comparing josh/cargo-nextest (3b0eaf4) with master (7d574a9)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	josh/cargo-nextest	Change
❌	document_symbol	4.3 ms	5.3 ms	-17.77%

[sdankel]

Nice! If this works out, we could try it for #6509 as well.