underscores are ok in test case names

I've been sitting on this branch for more than half a year, 
it's time to finally kiss the :frog: of PR'ing it.

We have this convention with test function names that they should be  snake-cased. 
Each and every time I commit a test case, the code sniffer jumps at me. 

This change allows snake case for functions *if* they have the `@test` annotation.

@rebuy-de/it-development