selftests/bpf: Add test for skb_load_bytes

Use bpf_prog_test_run_opts to test the skb_load_bytes function. Tests
the behavior when offset is greater than INT_MAX or a normal value.

Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220416105801.88708-4-liujian56@huawei.com