tests: make test-execute pass on openSUSE
    
    In my understanding user group "3" (aka "sys") is kept for historical reasons
    but not really useful these days. That's probably explained why this group
    isn't defined on openSUSE.
    
    Hence let's drop reference to this user group, this shouldn't lessen the
    revelance of the test since SupplementaryGroups= is still tested with 2 other
    groups.
    
    (cherry picked from commit d723b0467d7b8c5c772086d5352442f3fca4368d)