found a bug in dkg_test

[Mahsa-Bastankhah]

I added an additional condition to line because the previous version was ignoring sending responses to some old nodes that it is not supposed to ignore. This if branch should be only triggered for nodes in common between the old and new committee but apparently dkg.nidx for some old nodes has some arbitrary values and this activates the branch so I added one more condition to make sure that this branch is only triggered for the last old dkg node which is part of the new dkg as well.