selftests/bpf: Add kfree_skb raw_tp test

Load basic cls_bpf program.
Load raw_tracepoint program and attach to kfree_skb raw tracepoint.
Trigger cls_bpf via prog_test_run.
At the end of test_run kernel will call kfree_skb
which will trigger trace_kfree_skb tracepoint.
Which will call our raw_tracepoint program.
Which will take that skb and will dump it into perf ring buffer.
Check that user space received correct packet.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191016032505.2089704-12-ast@kernel.org