selftests/bpf: Mark SYN cookie test skipped for UDP sockets

SYN cookie test with reuseport BPF doesn't make sense for UDP sockets. We
don't run it but the test_progs test runner doesn't know about it. Mark the
test as skipped so the test_progs can report correctly how many tests were
skipped.

Fixes: 7ee0d4e9 ("selftests/bpf: Switch reuseport tests for test_progs framework")
Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200212103208.438419-1-jakub@cloudflare.com