Move unread separator off-screen in pill snapshot fixture

The fixture put the separator at index 0 of messageItems, which with
reverseLayout = true sits at the bottom of the viewport. The pill hides
itself while the separator is visible, so the snapshot did not exercise
the pill. Place the separator at the end of the list so it stays above
the visible area.

Author: andremion

stream-chat-android-compose/src/test/kotlin/io/getstream/chat/android/compose/ui/messages/MessageListTest.kt

@@ -260,7 +260,7 @@ private val TwoMessagesListState = MessageListState(
 )
 
 private val UnreadLabelMessageListState = LoadedMessageListState.copy(
-    messageItems = listOf(UnreadSeparatorItemState(unreadCount = 9)) + LoadedMessageListState.messageItems,
+    messageItems = LoadedMessageListState.messageItems + UnreadSeparatorItemState(unreadCount = 9),
     unreadLabel = MessageListController.UnreadLabel(
         unreadCount = 9,
         lastReadMessageId = PreviewMessageData.message1.id,