autopkgtest-novirt: run tests with unprivileged user

The ruby2.* testsuite is meant to be run as an unprivileged user, and has failing tests when run as root.
The tests doesn't have needs-root.

AFAICS autopkgtest-novirt runs tests from a root user, without -u salsaci or equivalent.
Example failure: https://salsa.debian.org/lts-team/packages/ruby/-/jobs/5989809

Related to #9, I suppose :)