MM-16949 Fix posts returned from unread API when the two different list contains parent post and comments (#11617)

* fix posts returned from unread API when the list contains parent post/s

* add ExtendAll to PostList and update test per feedback

* revert unintentional change to the other test and fix comment

* update the existing postlist.Extend, filter unique values and update unit tests

api4/post_test.go

@@ -1717,11 +1717,17 @@ func TestGetPostsForChannelAroundLastUnread(t *testing.T) {
 	post3 := th.CreatePost()
 	post4 := th.CreatePost()
 	th.CreatePost() // post5
-	post6 := th.CreatePost()
-	post7 := th.CreatePost()
-	post8 := th.CreatePost()
-	post9 := th.CreatePost()
-	post10 := th.CreatePost()
+	replyPost := &model.Post{ChannelId: channelId, Message: model.NewId(), RootId: post4.Id, ParentId: post4.Id}
+	post6, resp := Client.CreatePost(replyPost)
+	CheckNoError(t, resp)
+	post7, resp := Client.CreatePost(replyPost)
+	CheckNoError(t, resp)
+	post8, resp := Client.CreatePost(replyPost)
+	CheckNoError(t, resp)
+	post9, resp := Client.CreatePost(replyPost)
+	CheckNoError(t, resp)
+	post10, resp := Client.CreatePost(replyPost)
+	CheckNoError(t, resp)
 
 	// All returned posts are all read by the user, since it's created by the user itself.
 	posts, resp := Client.GetPostsAroundLastUnread(userId, channelId, 20, 20)