Add native testing support (pcap/C)

This patchs adds native testing support. It runs testing cases with
native performance (pcap) avoiding binding overhead.

* src/Makefile
* src/pf_test_native.c. native support using pcap in C lang
* src/pf_test_native.lua. string handling and ts driving

$ make
gcc -o pf_test_native pf_test_native.c -lpcap
$ make test_native
luajit -l pf_test_native -e 'pf_test_native.run()'

[*] Running test plan: ts/tests/0001_plan.ts

enabled tests: 11 of 12

[...]
Test case running ...
id: 6
description: udp dst port test
filter: udp dst port 2087
pcap_file: ws/tftp_wrq.pcap
expected_result: 49
enabled: true
tc id 6 PASS
tc id 6 AVG ET  0.000026
[...]