[CI] Tune GPU resources for test

[tjtanaa]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

There are test groups that does not require multi-gpu agent instance.

mimo test uses custom ci configuration which only uses 1 GPU
str(Path(__file__).parent.parent / "stage_configs" / "mimo_audio_ci.yaml"),

Test Plan

https://buildkite.com/vllm/vllm-omni/builds/5572/steps/canvas

Test Result

test-ready.yml: https://buildkite.com/vllm/vllm-omni/builds/5572/steps/canvas
test-merge.yml: - label: "Engine Test" is exactly the same as in test-ready.yml.

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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. Please provide the test scripts & test commands. Please state the reasons if your codes don't require additional test scripts. For test file guidelines, please check the test style doc
• The test results. Please paste the results comparison before and after, or the e2e results.
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model. Please run mkdocs serve to sync the documentation editions to ./docs.
• (Optional) Release notes update. If your change is user-facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)