Things I Learnt The Hard Way - Good Languages Come With Tests

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.

The reason is kinda obvious on this one: When the language itself brings a testing framework, it reduces the friction for people to start writing tests, and that includes the authors of the language itself and the community.

Sure, better frameworks may come along, and languages that don't have a testing framework in their standard library may have options with better support and easier access but, again, when they are there from the start, the start is better and the final result is better.