On my machine, everything works. It's on the testers' machines that I have a problem and I don't know if there is a simple way to mimic what they are doing that causes the test failures.

My suggestion was aimed precisely at letting you mimic a situation similar to that on the testers' machines, at least for the scenario described in your excerpt.

Did you try running the testsuite with the setup I suggested? You should at least be able to show that it would have highlighted the one problem you already found and fixed.

If there are other failure modes you want to test for, you will first need to identify those.