No replies here, just some more ideas to consider:
better make the test something directly useful to the developers themselves, something they can use to improve themselves with, rather than a mere gauge.
pair programming or peer review between your developers, e.g. selected non-trivial scripts. And improve code quality & skills faster, thus resulting in less debugging time in the long run. With an requires an explicit decision from management to support this kind of investment of time.
having some senior architect or lead would be helpful. Or consider asking each developer to dedicate some extra time for testing & coding in order to become the senior/expert for a certain category of modules or usage scenario. Maybe ask them for (short mini) talks about their areas of expertise + maintained resources & handouts in a wiki.