test: add test asserting partition is not empty

[michaelhtm]

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michaelhtm

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@michaelhtm: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
s3-verify-attribution	8c603ccf785cc85c0497a4455aa155489a9e667e	link	false	/test s3-verify-attribution
s3-metadata-file-test	8c603ccf785cc85c0497a4455aa155489a9e667e	link	true	/test s3-metadata-file-test
s3-unit-test	8c603ccf785cc85c0497a4455aa155489a9e667e	link	true	/test s3-unit-test
s3-recommended-policy-test	8c603ccf785cc85c0497a4455aa155489a9e667e	link	true	/test s3-recommended-policy-test
s3-kind-e2e	8c603ccf785cc85c0497a4455aa155489a9e667e	link	true	/test s3-kind-e2e
s3-release-test	8c603ccf785cc85c0497a4455aa155489a9e667e	link	true	/test s3-release-test

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