DAOS-17309 test: unit test for filling single bucket

[NiuYawei]

VOS unit test for filling single bucket with single array object.

Steps for the author:

• [ ] Commit message follows the guidelines.
• [ ] Appropriate Features or Test-tag pragmas were used.
• [ ] Appropriate Functional Test Stages were run.
• [ ] At least two positive code reviews including at least one code owner from each category referenced in the PR.
• [ ] Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• [ ] Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions[bot]]

Ticket title is 'Review VOS durable format for potential features' Status is 'In Progress' Labels: 'md_on_ssd,md_on_ssd2' https://daosio.atlassian.net/browse/DAOS-17309

[NiuYawei]

Landed in different PR, close this one.