Manufacturing test automation software. It's a niche corner of professional software. If your stuff goes down, you don't make product. Downtime results in huge sums of potential profit being lost very rapidly. It's exciting when it goes right and downright crushing when it goes wrong. If you do it right you can make huge gains in process efficiency.
Automating functional tests for products and their subassemblies while on the manufacturing line.
We do write tests for our software too though. The projects are written in golang, unittests sprinkled in with go's builtin test framework. Integration tests use our in house product simulations that were originally designed for firmware validation.