[tinnet]

Re #18: It exists, it's called mutation testing, and it's awesome (I only know the Java variant called http://pitest.org)

[epmatsw]

Stryker is a JS version. Google has some interesting whitepapers about their usage of mutation testing as well: https://ai.google/research/pubs/pub46584

[r-s]

Ruby, too: https://github.com/mbj/mutant I think most languages have something like this now.