I guess you could do A/B testing with feature flags but honestly, it won't be any good. If you want a good answer if to go with A or B, you want to have many data points, not just from one specific customer.

Gradual rollout and specific customer opt-in to certain features can certainly be done with feature flags, I wouldn't call that A/B testing unless you're running experiments and reaching a conclusion that A is better than B (or vice-versa).