Move search download tests into separate files

[katybaulch]

Description

When working on PDCT-1274, I found that the search test files are huge and kinda grim to maintain. This PR splits the download search tests into separate files as part one of a refactor to make the search tests more maintainable.

Proposed version

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Type of change

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• Bug fix
• New feature
• Breaking change
• GitHub workflow update
• Documentation update
• Refactor legacy code
• Dependency update

How Has This Been Tested?

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Reviewer Checklist

• DB_CLIENT DEPENDENCY IS ON THE LATEST VERSION
• The PR represents a single feature (small driveby fixes are also ok)
• The PR includes tests that are sufficient for the level of risk
• The code is sufficiently commented, particularly in hard-to-understand areas
• Any required documentation updates have been made
• Any TODOs added are captured in future tickets
• No FIXMEs remain