Yes, a comprehensive nuclear test ban treaty reduces the risks to the environment, because each nuclear test is very destructive. Nuclear weapons are extremely harmful. That is why they are powerful weapons. With fewer testing, there is less destruction, and the environment is kept in better shape. With as destructive as nuclear weapons are, this is a valid goal.
Yes, a nuclear test ban reduces risks to the environment, because nuclear weapons are very environmentally damaging. Even if a nuclear weapon is not used in warfare, that it is deployed at all carries a significant risk to the environment, even on a global scale. Banning tests reduces deployment, which benefits the environment.