selftests: bpf: add selftest for __sk_buff context in BPF_PROG_TEST_RUN

selftests: bpf: add selftest for __sk_buff context in BPF_PROG_TEST_RUN

Simple test that sets cb to {1,2,3,4,5} and priority to 6, runs bpf
program that fails if cb is not what we expect and increments cb[i] and
priority. When the test finishes, we check that cb is now {2,3,4,5,6}
and priority is 7.

We also test the sanity checks:
* ctx_in is provided, but ctx_size_in is zero (same for
  ctx_out/ctx_size_out)
* unexpected non-zero fields in __sk_buff return EINVAL

Signed-off-by: Stanislav Fomichev <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>