Fix stale steward pagination tests

[luch91]

Summary

• add the missing steward permission fixture required by steward submission pagination tests
• use timezone-aware submission dates in the pagination test setup

Verification

• python3 manage.py test contributions.tests.test_pagination -v 2
• verified in the repo's Python 3.13 Docker environment

[JoaquinBN]

Same as the other test fixes — valid catches, @luch91. We'll handle these as part of internal test suite maintenance. Once our current review and rebranding wraps up, we'll organize the repo with proper issues for contributions. Thanks for looking into it and hope to see you back!

[luch91]

@JoaquinBN This makes sense. Glad the issues were helpful. I’ll keep an eye out when contribution issues are up