dns: check IPv4 DNS after reapplying on disabled IPv6
The NM upstream commit cb292445 introduced a bug, which made the DNS
configuration exported via D-Bus is empty after reapplying on disabled
IPv6, therefore the auto IPv4 DNS server will be purged by mistake.
And the NM upstream commit 0a029951 fixed the issue. In order to
validate the expected behavior in this scenario, add NMCI test
dns_reapply_on_disabled_ipv6.