libbpf: add support for ctx_{size, }_{in, out} in BPF_PROG_TEST_RUN

Support recently introduced input/output context for test runs.
We extend only bpf_prog_test_run_xattr. bpf_prog_test_run is
unextendable and left as is.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>