[CIVIS-1677] TST Optimize tests by removing marks and unneeded sleep calls

[gkcivis]

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed

Remove unneeded time.sleep call in test_polling.py. Remove pytest.mark calls in test_io.py.