test: verify vmnet accepts partial DHCP range

[nirs]

When using --end-address smaller than the last host address in the subnet (e.g. 192.168.200.200 with /24 mask), vmnet accepts the configuration and both gateway and NAT traffic work normally.

This is useful for reserving addresses above --end-address for static IP assignment via /etc/bootptab, for example to give a VM a stable IP that won't be handed out to other VMs by DHCP.

Example:

% sudo pytest -xvs -k test_partial_dhcp_range
...
root: interface: {'vmnet_end_address': '192.168.200.200',
    'vmnet_start_address': '192.168.200.1', ...}
test: Sent ARP request to 192.168.200.1
test: Got ARP reply in 0.002440 seconds
test: Sent ICMP echo request to 192.168.200.1
test: Got ICMP reply in 0.000574 seconds
test: Sent ICMP echo request to 1.1.1.1
test: Got ICMP reply in 0.002712 seconds
PASSED