tests: use network config to generate unique IPs
tests: use network config to generate unique IPs

Using DEFAULT_HOST_IP and DEFAULT_GUEST_IP expose us to same MAC
address already used error when creating new network interfaces besides
the default one.

Details:
MAC addresses used to configure a network interface are computed based
on the guest IP address used as well for network interface configuration.
When we attach a new network interface, we usually generate two unique IPv4
addresses, using the `UniqueIPv4Generator`. This might generate the
DEFAULT_GUEST_IP as the next unique IPv4 address, and hence, when
trying to set up this new network interface for a microVM, the test will
return with error, since it can not attach two network interfaces with the
same MAC address.

Signed-off-by: Iulian Barbu <3d73465fd48f87db8f58a730f2d3c3f04dff43ae@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker