support: Don't fail on fchown when spawning sgid processes

In some cases (e.g. when podman creates user containers), the only other
group assigned to the executing user is nobody and fchown fails with it
because the group is not mapped.  Do not fail the test in this case,
instead exit as unsupported.

Reported-by: Frédéric Bérat <fberat@redhat.com>
Tested-by: Frédéric Bérat <fberat@redhat.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>