selftests, bpf: Add test for veth native XDP

Add a test case for veth native XDP. It checks if XDP_PASS, XDP_TX and
XDP_REDIRECT work properly.

  $ cd tools/testing/selftests/bpf
  $ make \
  	TEST_CUSTOM_PROGS= \
  	TEST_GEN_PROGS= \
  	TEST_GEN_PROGS_EXTENDED= \
  	TEST_PROGS_EXTENDED= \
  	TEST_PROGS="test_xdp_veth.sh" \
  	run_tests
  TAP version 13
  1..1
  # selftests: bpf: test_xdp_veth.sh
  # PING 10.1.1.33 (10.1.1.33) 56(84) bytes of data.
  # 64 bytes from 10.1.1.33: icmp_seq=1 ttl=64 time=0.073 ms
  #
  # --- 10.1.1.33 ping statistics ---
  # 1 packets transmitted, 1 received, 0% packet loss, time 0ms
  # rtt min/avg/max/mdev = 0.073/0.073/0.073/0.000 ms
  # selftests: xdp_veth [PASS]
  ok 1 selftests: bpf: test_xdp_veth.sh

Signed-off-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>