test: add test asserting partition is not empty

[michaelhtm]

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[ack-prow]

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This pull-request has been approved by: michaelhtm

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[ack-prow]

@michaelhtm: The following test 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	8c603cc	link	false	/test s3-verify-attribution

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