Follow-up for 2b07a3211b.
Fixes the failure found in
https://autopkgtest.ubuntu.com/results/autopkgtest-noble-upstream-systemd-ci-systemd-ci/noble/amd64/s/systemd-upstream/20241115_182040_92382@/log.gz
. Relevant logs:
```
Nov 16 02:48:36 systemd-networkd[2706]: veth99: Reconfiguring with /run/systemd/network/25-dhcp-client-ipv6-only.network.
Nov 16 02:48:36 systemd-networkd[2706]: veth99: NDISC: Started IPv6 Router Solicitation client
Nov 16 02:48:36 systemd-networkd[2706]: veth99: IPv6 Router Discovery is configured and started.
Nov 16 02:48:36 systemd-networkd[2706]: veth99: NDISC: Sent Router Solicitation, next solicitation in 3s
Nov 16 02:48:36 systemd-networkd[2706]: veth99: NDISC: Received Router Advertisement from fe80::1034:56ff:fe78:9abd: flags=0xc0(managed, other), preference=medium, lifetime=30min
Nov 16 02:48:36 systemd-networkd[2706]: veth99: NDISC: Invoking callback for 'router' event.
Nov 16 02:48:36 systemd-networkd[2706]: veth99: link_check_ready(): dynamic addressing protocols are enabled but none of them finished yet.
Nov 16 02:48:36 systemd-networkd[2706]: veth99: DHCPv6 client: Starting in Solicit mode
Nov 16 02:48:36 systemd-networkd[2706]: veth99: DHCPv6 client: State changed: stopped -> solicitation
Nov 16 02:48:36 systemd-networkd[2706]: veth99: Acquiring DHCPv6 lease on NDisc request
Nov 16 02:48:36 systemd-networkd[2706]: veth99: DHCPv6 client: Sent Solicit
Nov 16 02:48:36 systemd-networkd[2706]: veth99: DHCPv6 client: Next retransmission in 1s
Nov 16 02:48:37 systemd-networkd[2706]: veth99: DHCPv6 client: Sent Solicit
Nov 16 02:48:37 systemd-networkd[2706]: veth99: DHCPv6 client: Next retransmission in 1s
Nov 16 02:48:39 systemd-networkd[2706]: veth99: NDISC: Received Neighbor Advertisement from fe80::1034:56ff:fe78:9abd: Router=yes, Solicited=yes, Override=no
Nov 16 02:48:39 systemd-networkd[2706]: veth99: NDISC: Invoking callback for 'neighbor' event.
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 client: Processed Reply message
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 client: T1 expires in 50s
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 client: T2 expires in 55s
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 client: Valid lifetime expires in 2min
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 client: State changed: solicitation -> bound
Nov 16 02:48:39 systemd-networkd[2706]: veth99: DHCPv6 address 2600::15/128 (valid for 1min 59s, preferred for 1min 59s)
Nov 16 02:48:41 systemd-networkd[2706]: veth99: Received updated DHCPv6 address (configured): 2600::15/128 (valid for 1min 58s, preferred for 1min 58s), flags: no-prefixroute, scope: global
Nov 16 02:48:41 systemd-networkd[2706]: veth99: DHCPv6 addresses and routes set.
Nov 16 02:48:41 systemd-networkd[2706]: veth99: link_check_ready(): IPv4LL:no DHCPv4:no DHCPv6:yes DHCP-PD:no NDisc:no
Nov 16 02:48:41 systemd-networkd[2706]: veth99: State changed: configuring -> configured
```
The interface veth99 entered the configured state after 5 seconds, but
at the same time, the `wait_online()` in the test script considered the
test failed.
The function `wait_online()` first invokes
`systemd-networkd-wait-online` with `--timeout=20`, then check setup
states of interfaces with 5 seconds timeout. So, the failure suggests
that `systemd-networkd-wait-online` finishes immediately, as the state
file was not updated when it is invoked, and thus it handles the
interface veth99 already in the configured state.
2e5b0412f9
