Don't all of "serious" programming languages meet that bar? Java, C#, Go, Python, etc all have enormous test suites. Once you get into the third party, things become much more uneven, but if you can restrict yourself to say the top N packages in a language, those are going to have better than average development practices which makes that plausible.

> Once you get into the third party

year, that's usually what is referred as ecosystem.