Hi Alexander,
I see your answer on gmane, but not by e-mail, strange.
But thanks for the tip, it worked and I got oe-selftest -r runtime_test.TestImage.test_testimage_dnf to run successfully.
(for future readers: by following "Yocto Project Development Tasks" manual section "Performing Automated Runtime Testing" https://docs.yoctoproject.org/dev-manual/common-tasks.html#enabling-runtime-tests-on-qemu)