Enable integration tests for wait-online logic

[slyon]

Description

Follow-up to #535 which can only be landed after the upstream Netplan CI got upgraded to systemd v258. Or Noble received backports of the relevant wait-online changes, as landed in https://launchpad.net/ubuntu/+source/systemd/257.2-3ubuntu1

Checklist

• Runs make check successfully.
• Retains 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]

needs to be rebased on top of #552

[JegadishD]

@slyon
Major part of this PR is already merged and taken care by your earlier work - https://github.com/canonical/netplan/pull/566/changes
The only pending change to be merged from this PR is the change to enable to networkd-wait-online.service (tests/integration/ethernets.py)

Can you please update the PR or please let me know, I am open to update it from my end.

[slyon]

@JegadishD Thanks for getting the CI updated to 26.04, unblocking this additional test!

Please feel free to move ahead with this yourself. IIRC all it needs should be a cherry-pick of that single commit, which we're currently carrying as a distro-patch in Debian/Ubuntu: https://sources.debian.org/src/netplan.io/1.2.2-1/debian/patches/0001-tests-ethernets-enable-wait-online-intergration-test.patch

[JegadishD]

@slyon Thanks for the reply. I have updated the PR with only the specific change required. The GitHub workflows are all passing. I shall be merging it shortly!