I've been impressed with code written by Fable in my testing:
Difftastic: found small optimisations in a hot loop I'd already profiled extensively. Helped me prototype Dijkstra to A* too (hard to find a good heuristic).
Garden: Found some real bugs in my simplistic typechecker.
LLVM's lit tool for testing files is the most successful testing DSL I've ever seen. It's used a decent number of projects outside of LLVM.
By contrast I don't see many folks using e.g. Cucumber these days.
Do any tech streamers try new software live? It'd be a really fun way of doing UX testing.