Update the builder test files (utils/test-rh8, utils/test-rh9) to allow for use without constant editing.

The test scripts `utils/test-rh8` and `utils/test-rh9` could use some love:

- Change the names to `utils/test-el8` and `utils/test-el9` respectively
- Refactor the functions so that the builder image's `org/repo:tag` can be injected at runtime.
- More, as we see fit or as needed to make these usable by others.