In the recent issue MDL-56225, we started to record the current user as
the usermodified in the forum_discussions table when updating a forum
post. It made sense but it was a mistake.

Even if the current user really modifies the discussion by updating the
post, the field usermodified has actually been always interpreted and
displayed as the last post' author. Not as the last user who touched the
discussion.

This patch reverts that particular change to the previous behaviour and
adds explicit unit test for it.