test: ensure prefetched SRPMs are included in source image for multi-arch builds

[tkdchen]

Description

This PR extends a test of multi-platform builds to have a sanity check, which ensures prefetched SRPMs are included in the built source container image.

Issue ticket number and link

STONEBLD-2454

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added meaningful description with JIRA/GitHub issue key(if applicable), for example HASSuiteDescribe("STONE-123456789 devfile source")
• I have updated labels (if needed)

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[openshift-ci]

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[tkdchen]

Rebased. Conflicts are fixed.