This is what U.S., Japan or South Korea could do.  Take a drone, sanitize it so that there is no way to identify its source, load it with very high explosives, fly it into North Korea under radar detection, take out the test site and then claim a mishap of some sort at the test site.  Missiles could destroy the test site, but would be detected incoming by radar and their source might be identified.  You have to think outside the box in such situations.