SwNodeIndex should really be final

Change-Id: I2ae917b43b18b14dce01121902a678d45ad520fe
fdo#85876: add unittest

rather uglyish unittest, but still worth it as cutting nothing needing
to have sideeffects is really kind of non-obvious and prone to return

Change-Id: I02bbfb05f019874d873670cdae060ac3183f5ca6