selftests/bpf: check insn processed in test_verifier

Teach test_verifier to parse verifier output for insn processed
and compare with expected number.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Edward Cree <ecree@solarflare.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>