Items that are in your house at the Server Save on which the backup is created will not be available for you during the Test Server. CipSoft usually tell players in advance about the TS backup, so you can move items you want to have during the Test to your depot to ensure you'll have them there.
For example, for the 2021 SU the backup was taken at the Server Save of May 25 and a news ticker was posted one week in advance.
You can even do the same for Store products, as long as you bid on a house on the first day of the Test and win the auction, which will allow you to use the house as in the live game worlds.