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.
• check the coders at work thread for more ideas: Questions from "Coders at Work"

HTH,
Peter