As you've discovered, good automated testing is mandatory for iterative development. If execs tell you to do something unprofessional, it's ok to say no. Indeed, if we're going to think of ourselves as professionals, I think it's mandatory.
But one developer quitting rather than do bad work does change something. At the very least, it means that the developer can find another, better job. But it's also an opportunity for companies to learn. And for those who don't, good people refusing to prop up bad companies is a step forward.
I'd also add there's a a lot that can happen between refusing to do bad work and quitting/getting fired. Saying no is the beginning of a negotiation. A negotiation that won't happen if we just say yes all the time.
