At https://doc.qt.io/qt-5/qmake-common-projects.html#building-a-testcase there's a documented way to pass additional test arguments via Makefile variables TESTRUNNER and TESTARGS.
That's currently missing in the CMake port.
We could perhaps implement something similar via environment variables instead, because ninja doesn't have Ninja variables afaik.
This also implies that we need to generate a wrapper shell script that needs to be used to run the tests. Currently we execute them directly.