Fix test failures due to collecting 0 tests (#485)

## Summary

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Include a short paragraph of the changes introduced in this PR.
If this PR requires additional context or rationale, explain why
the changes are necessary.
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Fixes the current E2E failures on main. See #482 for more details.

## Test Plan

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Ran CI with patch applied; [result
here](https://github.com/vllm-project/guidellm/actions/runs/19650016859/job/56274540060#step:7:195).

## Related Issues

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- Resolves #482 

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: sjmonson

.github/actions/run-tox/action.yml

@@ -23,7 +23,7 @@ runs:
       shell: bash
     - name: Run tox
       run: |
-        tox run -e "${{ inputs.tox-env }}" -- ${{ inputs.tox-args }}
+        tox run -e "${{ inputs.tox-env }}" -- ${{ inputs.tox-args }} || ret=$?
         # errno 5 means all tests were filtered out, ignore
-        ret=$? && [[ $ret -eq 5 ]] && exit 0; exit $ret
+        [[ "${ret:=0}" -eq 5 ]] && exit 0; exit $ret
       shell: bash