refactor make_materials*() helpers

[jku]

The way we build test cases has changed a bit through the lifetime of this project so the current helpers are not as useful as they could be -- there's likely also unused code in there.

• Practically all verification tests are now implemented in their own directories in test/assets/bundle-verify (directory contains a bundle, optional trust root, and the expected result is encoded in directory name)
• The remaining tests use varying mechanisms of material collection

I think we could refactor the make_materials*() helper methods so that they handle this directory-per-test-case and then build all remaining test cases in that way too.