refactor: ci: Overhaul test script for `filedma` samples

These changes came along in the process of implementing regression testing based on hashes of the output files.

As such, I'm splitting that MR ( !83 (merged)) into a refactor and the added functionality.

Bonus: the shell script attempts to adhere to Google's shell style guide: https://google.github.io/styleguide/shellguide.html