Tests for lwIP's sock_udp port

This tests the sock_udp port of lwIP. There is no network device needed since a virtual device is provided at the backend.

These tests test both IPv4 and IPv6 capabilities. They can be activated by the LWIP_IPV4 and LWIP_IPV6 environment variables to a non-zero value. IPv6 is activated by default:

make all test
# or
LWIP_IPV6=1 make all test

To just test IPv4 set the LWIP_IPV4 to a non-zero value (IPv6 will be deactivated automatically):

LWIP_IPV4=1 make all test

To test both set the LWIP_IPV4 and LWIP_IPV6 to a non-zero value:

LWIP_IPV4=1 LWIP_IPV6=1 make all test

Since lwIP uses a lot of macro magic to activate/deactivate these capabilities it is advisable to test all three configurations individually (just IPv4, just IPv6, IPv4/IPv6 dual stack mode).