Revert "images/tests: Globally-writeable /etc/passwd"
Revert "images/tests: Globally-writeable /etc/passwd"

This reverts commit ca35cd633a46d2156ef598e827ce230391684926, #22592.

As described in that commit message, the access was broadened to allow
ssh from containers launched from the tests image.  But since
openshift/release@7baa9f2e44 (ci-operator/templates/openshift: Remove
Terraform-state-based SSH gathers, 2020-01-25, openshift/release#6854)
landed, we no longer need to SSH from those containers.  Restore the
usual access restrictions to address CVE-2019-19347[1].

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1793287

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