> You can be sure that if a language brings a testing framework -- even minimal -- in its standard library, the ecosystem around it will have better tests than a language that doesn't carry a testing framework, no matter how good the external testing frameworks for the language are.

D not only has it as a library, it's a core feature of the language. As incongruous as it sounds, builtin unittests have been utterly transformative to how D code is written. Built in documentation generation has had the same effect.