test-systemd-tmpfiles: skip when /tmp has unexpected ownership

test-systemd-tmpfiles: skip when /tmp has unexpected ownership

The systemd-tmpfiles binary will report a fatal error if /tmp is not owned
either by root, or by the current user:

  Detected unsafe path transition /tmp (owned by nobody) →
    /tmp/test-systemd-tmpfiles.a8qc6n18 (owned by berrange)
    during canonicalization of
    tmp/test-systemd-tmpfiles.a8qc6n18/test-content.7chd7rdi

When doing development inside a 'toolbox' container (which is required
on a Fedora SilverBlue distro), /tmp is owned by 'nobody', because it
has been passed through from the host and host UID 0 gets mapped to
UID 65536 by usernamespaces. This triggers the unsafe path transition
error message.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>