[IMP] create_pcap_when_test_started: added a little comment :)
[IMP] create_pcap_when_test_started: added the interface argument
[IMP] stop_pcap_when_test_completed.sh: kill all pids contained in .tcpdump-stop
[IMP/FIX] stop_pcap_when_test_completed.sh: pids are in .tcpdump-pid (and not .tcpdump-stop) + remove .tcpdump_pid after having killed all processes in the file