Thank you both for your answers.
did not know about the status handling. I really like that idea.
I just heard from our SAP Basis, that we are going to have a sandbox with PO connection next year (cause then we are about to migrate from PI to PO). Then we will create all our test interfaces there. However, the solution for now is that we will create a transport request and let it stay in our development system. The interfaces will be in a certain namespace and package to indicate that they are test interfaces and will use status "In development".