Custom project to run tests

Using a simple gitlab ci setup, what would be the ‘best’ or safest way to include a custom repo with test code? In a group with several repos (all from the same template) I would like to include private code from just one repo in all of them for testing purposes. I’ve considered cloning it manually but it looks to me like the safest way to do that is using deploy keys or adding the testing code as a submodule I’d rather not do either of these things and possibly use the credentials granted to the runner to clone the repo locally (all projects are in the same group).

If it matters at all I’m using the docker image julia:1.5.3-buster
I don’t however think it should. The way the testing works, the output of the actual program gets piped in to the test program which then makes a few calls to apis and wepbages before it compares results and prints out a success or failure message.

Maybe there’s just a drastically better way of doing this, I’m not sure.

Thanks in advance for any help or suggestions,