Fix a flaky content collection test

[delucis]

Changes

• Updates and simplifies a test that was flaky
• Example failing run of the old test: https://github.com/withastro/astro/actions/runs/21918847401/job/63293530577?pr=15475#step:8:5262

Testing

• Yup

Docs

N/A

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Latest commit: 1aa7490

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