fix: cleanups in translate test discovery

[romanc]

Description

Some refactors/cleanups in the code that discovers translate tests. Follow-up from PR #403 where I messed up the nice error message that we throw in case the translate test is not found.

How has this been tested?

All good as long as CI is still green: we use the translate test discovery system extensively in the CI.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. add new modules to docs/docstrings/): N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A

[FlorianDeconinck]

LGTM