test-network: increase wait_online timeout to handle longer dhcpv4 transient timeout

test-network: increase wait_online timeout to handle longer dhcpv4 transient timeout

Previous commits changed the dhcpv4 retransmission algorithm to be
slightly slower, changing the amount of time it takes to notify
systemd-networkd that the dhcpv4 configuration has (transiently)
failed from around 14 second up to 28 seconds.

Since the test_dhcp_client_with_ipv4ll_without_dhcp_server test
configures an interface to use dhcpv4 without any operating dhcpv4
server running, it must increase the amount of time it waits for
the test interface to reach degraded state.