This question is one that many people have been asking for a very long time because the subject matter is not one that is easily answered. There is a large group of people that believe the answer is simple and a quick resolution can be had. But that is not the case in this situation. Japan should test their rockets.
I think that every government has the necessity to have defenses in place just as the US does. If they choose to test them, that is their own agenda. It doesn't mean they are going to bomb anyone. Who are we to say they can or can't make their own weapons and test them.
I definitely think that South Korea and Japan should test rocket launches just like North Korea did. If a rogue nation like North Korea is going to test its missiles, then it is only logical for countries that are in the region to do dthe same to make sure their defenses work.
Japan and South Korea do not need to test missiles the way North Korea does. North Korea does not have the technology that their neighbors to the south do. That does not mean North Korea Can't hit either of those countries with their missiles. It just means,Japan and South Korea have better ways to test their missiles, where they do not scare most of the world.
No, South Korea and Japan should not test rockets as North Korea did, because North Korea already knows that these nations have these weapons capabilities. South Korea and Japan do not need to show off that they have these powers. They should just let North Korea be crazy and flex their muscles as they want to.