Hi Femke,
create a new project in 7.0.2, and import the module.
Then you close the project, open 7.23.x and open the project from disk. It will ask if you want to convert/upgrade. Do so.
Take the same steps to go to 8.x.x
Now you can export the module and import it in your project.
In addition to the previous comment, the Unit Testing module has Platform Support so you can also raise a support ticket directly with Mendix.
Hopefully this is on their roadmap to fix soon.