bpf: add various test cases for verifier selftest

Add couple of verifier test cases for x|imm += pkt_ptr, including the
imm += x extension.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>