tests: Add network filter tests to the check-qtest-s390x list

With some small modifications, we can also use the the netfilter,
the filter-mirror and the filter-redirector tests on s390x.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1502951113-4246-3-git-send-email-thuth@redhat.com>
Reviewed-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>