selftests: bpf: Add selftest for __sk_buff tstamp

Make sure BPF_PROG_TEST_RUN accepts tstamp and exports any
modifications that BPF program does.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191015183125.124413-2-sdf@google.com