tests: Wait for network *online* and multi-user targets

[roberth]

This should help prevent some test stalls.

By default, multi-user.target does not imply that the network is fully up.

Motivation

Make the tests more robust.

Context

• https://hydra.nixos.org/build/285626708

Relevant log lines, lots of omissions, but in order:

remote # [   15.628032] sshd-session[872]: Accepted publickey for root from 2001:db8:1::1 port 56514 ssh2: ED25519 SHA256:4ogA6akWufoJq7wuED/32LiLaVjB3+cw9gkbZ3/ugFk
remote # [   15.661248] sshd-session[872]: pam_unix(sshd:session): session opened for user root(uid=0) by (uid=0)
remote # [   16.800083] systemd[1]: Started Session 1 of User root.
remote # [   17.051388] sshd-session[892]: Received disconnect from 2001:db8:1::1 port 56514:11: disconnected by user
remote # [   17.067639] systemd-logind[703]: Session 1 logged out. Waiting for processes to exit.
client # [   18.222952] systemd[1]: Started DHCP Client.
client # [   18.232062] systemd[1]: Reached target Network is Online.
client # [   18.234925] systemd[1]: Reached target Multi-User System.
remote # [ 2489.293345] <irrelevant message, but note the timestamp>
timeout reached; test terminating...

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[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-01-20-nix-team-meeting-minutes-209/59119/1