[Bugfix] Fix test_mixtral_moe

[jeejeelee]

Purpose

Fix test_mixtral_moe on the CI, see: https://buildkite.com/vllm/ci/builds/29658#01991f9b-1ac1-49ce-8a30-dd6061787d8c/202-2418

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request addresses a failing test_mixtral_moe on the CI. The fix correctly introduces the dist_init fixture to manage the distributed environment for the test, which is a standard practice and should resolve the NCCL errors observed in the build logs. Additionally, a minor cleanup was performed in requirements/test.txt by removing a commented-out line. The changes are sound and directly address the issue.