[NFC] Update DXContainer tests to use fake parts

[NFC] Update DXContainer tests to use fake parts

The tests that are focused on testing the file structure should use
fake part names so that the errors get triggered consistently. As we
add parsing support for known data types and structures under the parts
the parsing errors change to be more semantically accurate.

This change ensures the general errors continue to work with less churn
on the test cases in the future.