test: refactor mix_version_test.

[siyuanfoundation]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

follow up #17531 to make code more consistent between 3.5 and main.

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[siyuanfoundation]

cc @ahrtr

[ahrtr]

lgtm

Thanks

[fuweid]

LGTM