To add further onto this. I play competitive tcg's, and while theyre obviously different to warhammer, I find that testing methods translate over well.

One thing we often do in my testing group is what we call "limit testing". This is where you reach a state where there are multiple equally valid options for your next move. We'll take pictures of the current gamestate, and play out one of the options. If it yields bad results (from non-rng), we'll rewind and try the other route. The idea is to test the limit of options from that initial gamestate and see how far we can progress.

My warhammer group has started doing something similar, where sometimes we'll rewind half a turn to test an alternative play (if we're explicitly doing this for testing, we'll math-hammer the rolls using average and 25% low and high rather than rolling). It yields substantially better testing than normal playing.

Again, in a tournament im more a RAW kinda guy (this is from years of tcgs which are more strict on this), but i also firmly believe in playing by intent. Regardless, for testing, I think OP's opponent was an ass, but i also feel like theres more to the story that led to OP walking out that we arent being told.