I can't imagine that you'll get banned for just forgetting to add your device as a Test Device. Plenty of developers make this mistake; it's nothing to be worried about.
The rule against fake impressions is most likely just to prevent someone from sitting there for hours, clicking on every ad that they're presented, and just raking in money for fake impressions.
If your account hasn't been suspended or you haven't been contacted by AdMob, I don't think you need to worry about it. If anything does happen, you can just tell them that you forgot to add your device as a Test Device. From what I could tell from AdMob's Help Center, if your account is disabled for Invalid Click Activity, you can repeal it at this link.