- The semi-colon seems odd with the "not only" because if it was a period, the first sentence wouldn't make much sense. Perhaps "Good test files ... code iterations, but they also serve as a functional specification."
- "This helps others ...  code. " -- not quite sure what you mean here? Are we speaking generally here or about DDH? If it's the latter I think we should say that good tests make sure IA's trigger when they should, not-trigger when they shouldn't and make sure edge cases are handled.

- "This, in turn" -> ", which, in turn" (removing the repeated "this")