Hi Thimo,
no, it is not possible to have local interfaces.
As Michal suggested you could try a sperate transport request that will never be released. Maybe put your test interfaces into a seperate namespace so that it is clear that those interfaces are only for testing purposes.
In addition ot that you could use the status handling in interface definition. You can set the status to "In Development" or "In Test". It will then not be possible to execute the interface in the productive system when it gets transported accidentially.
Best regards,
Verena