[MM-61466] Fix errcheck issues in server/channels/app/post_persistent_notification_test.go (#29207)

server/.golangci.yml

@@ -129,7 +129,6 @@ issues:
         channels/app/post.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_metadata.go|\
-        channels/app/post_persistent_notification_test.go|\
         channels/app/post_test.go|\
         channels/app/security_update_check.go|\
         channels/app/server.go|\