DeleteDraftComment: Don't bump 'lastUpdatedOn'

According to the implementation in change I3d8b13f70, posting a draft
inline comment to a change does not cause the change's 'lastUpdatedOn'
field to be updated.

However, the implementation of deleting a draft comment is inconsistent
with this, and does cause the 'lastUpdatedOn' to be updated.

Fix this inconsistency. Also add tests for both operations to confirm
that the timestamp is not updated.

Bug: Issue 4150
Change-Id: I222a702dd1dc529d4668a1dad6614a5839f63625

gerrit-server/src/main/java/com/google/gerrit/server/change/DeleteDraftComment.java

@@ -102,6 +102,7 @@ public class DeleteDraftComment
       setCommentRevId(c, patchListCache, ctx.getChange(), ps);
       plcUtil.deleteComments(
           ctx.getDb(), ctx.getUpdate(psId), Collections.singleton(c));
+      ctx.bumpLastUpdatedOn(false);
       return true;
     }
   }