selftests: Add nettest

Add nettest - a simple program with an implementation for various networking
APIs. nettest is used for tcp, udp and raw functional tests for both IPv4
and IPv6.

Point of this command versus existing utilities:
- controlled implementation of the APIs and the order in which they
  are called,
- ability to verify ingress device, local and remote addresses,
- timeout for controlled test length,
- ability to discriminate a timeout from a system call failure, and
- simplicity with test scripts.

The command returns:
  0  on success,
  1  for any system call failure, and
  2  on timeout.
Signed-off-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/Makefile

@@ -12,7 +12,7 @@ TEST_PROGS += udpgro_bench.sh udpgro.sh test_vxlan_under_vrf.sh reuseport_addr_a
 TEST_PROGS += test_vxlan_fdb_changelink.sh so_txtime.sh ipv6_flowlabel.sh
 TEST_PROGS += tcp_fastopen_backup_key.sh
 TEST_PROGS_EXTENDED := in_netns.sh
-TEST_GEN_FILES =  socket
+TEST_GEN_FILES =  socket nettest
 TEST_GEN_FILES += psock_fanout psock_tpacket msg_zerocopy reuseport_addr_any
 TEST_GEN_FILES += tcp_mmap tcp_inq psock_snd txring_overwrite
 TEST_GEN_FILES += udpgso udpgso_bench_tx udpgso_bench_rx ip_defrag