libsystemd-dhcp: Add a full DHCP address acquisition test case

libsystemd-dhcp: Add a full DHCP address acquisition test case

Upon startup the DHCP library will immediately send a DHCP Discover.
Handle the additional DHCP Discover checking and DHCP Offer sending
in test_addr_acq_recv_discover(). Write a zero UDP checksum, the
received XID and the faked MAC address into place to create a DHCP
message the library can accept.

Follow exactly the same steps when receiving a DHCP Request in
test_addr_acq_recv_request() and send the Ack. Check that the proper
addresses have been found by the DHCP library before exiting the
main loop and finishing the test.