SBOM test cleanup

[luhring]

The SBOM code is not in great shape and needs some work. So too do the tests for this code, which needs attention first, because without good tests, we can't confidently refactor the core business logic.

This PR makes progress on cleaning up the SBOM test code so that it's easier to work on it.

• testdata files are organized with paths that explain their role in the tests (APK vs. image SBOMs)
• Table-driven tests are used instead of duplicated test logic
• Unnecessary test structs are removed