DEV: attempts to make deleted spec not flakey (#21695)

It easier to check for presence in this case that to check for something not present, as depending on performance of the machine running the test this could take sometime to be changed and the test would fail.

plugins/chat/assets/javascripts/discourse/components/chat-message.hbs

@@ -62,7 +62,7 @@
         <div
           class={{concat-class
             "chat-message"
-            (if @message.staged "chat-message-staged")
+            (if @message.staged "chat-message-staged" "chat-message-persisted")
             (if @message.deletedAt "deleted")
             (if (and @message.inReplyTo (not this.hideReplyToInfo)) "is-reply")
             (if this.showThreadIndicator "is-threaded")

plugins/chat/spec/system/deleted_message_spec.rb

@@ -17,7 +17,9 @@ RSpec.describe "Deleted message", type: :system, js: true do
     it "shows as deleted" do
       chat_page.visit_channel(channel_1)
       channel_page.send_message("aaaaaaaaaaaaaaaaaaaa")
-      expect(page).to have_no_css(".chat-message-staged")
+
+      expect(page).to have_css(".chat-message-persisted")
+
       last_message = find(".chat-message-container:last-child")
       channel_page.delete_message(OpenStruct.new(id: last_message["data-id"]))