tests:ethernets: avoid flaky test_ip6_eui64 results

[slyon]

Description

Avoid flaky test_ip6_eui64 results, when statefull DHCPv6 is quicker than stateless EUI-64 address generation.
Also use a ra-stateless configuration option to dnsmasq, to avoid providing stateful DHCP addresses to the client;
ra-only doesn't seem to work properly, when combined with --dhcp-range (from integration/base.py).

This seems to fail from time to time on systemd v256 based systems, such as Debian testing or Ubuntu oracular.

Follow-up to #509

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

I've tested this locally multiple times in a Oracular LXD container (where it failed previously). Looking all good. The CI is green, too. Merging...