tests: make test-execute pass on openSUSE

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.