I agree the upcoming album may be a way bigger commercial success than TFOD. Testament's been working hard to reach new fans for the past couple of years. The last album was highly acclaimed and well promoted and the tours they've had have helped them reach new markets. In addition to that, they seem to have a well-thought promotional campaign planned for the new one. Nuclear Blast does a great job in promoting its bands and the success of TFOD ensured them that it pays to invest in Testament. The music will probably be even better too. Therefore, I'd be surprised if Dark Roots wasn't to sell better than its predecessor.
As for the sales data, the bands do have their sources. Some of the quotes in this threat prove that. Other thing is how accurate information they have. I agree that some of their albums (PWYP, Ritual, maybe SoB) have likely sold enough (or almost enough) to be certified gold, but due to the fact that SoundScan hadn't been introduced as late as in 1991 and since Atlantic isn't interested in applying for having them certified, Testament may never get the gold records for them.
Us, the fans do not have access to such information, unfortunately. There used to be a page called metal sludge a few years ago, which published "sludge scans" with information on album sales. Testament was included in these a few times. It hasn't been updated for 6 or 7 years, though.