Hi, I am not sure I understand, it sounds quite straightforward, if you have a small piece (1cm²) for testing and the entire role is 100m², then you can say you need 100.01 m² e.g. for the final product of 100m² (or you can make it a bit more sophisticated, you can define 1 piece of testing and the effort for cutting and testing it, and maybe a percentage of failures of the product), and add this to the 100m² role. openLCA does not necessarily "shout" if something does not make sense from a content point of view, unless the system can be calculated (which e.g. happens if you model that a process produces 2 kg of garments with input of 1 kg of garment and nothing else).
If you want, you can post a picture of the process you model (edit your question to be able to add images).