Use own version of assert().

gtests/net/packetdrill/assert.h

gtests/net/packetdrill/checksum.c

gtests/net/packetdrill/checksum_test.c

gtests/net/packetdrill/code.c

gtests/net/packetdrill/fmemopen.c

gtests/net/packetdrill/header.h

gtests/net/packetdrill/netdev.c

gtests/net/packetdrill/open_memstream.c

gtests/net/packetdrill/packet.c

gtests/net/packetdrill/packet.h

gtests/net/packetdrill/packet_parser.c

gtests/net/packetdrill/packet_parser_test.c

gtests/net/packetdrill/packet_socket_linux.c

gtests/net/packetdrill/packet_socket_pcap.c

gtests/net/packetdrill/packet_to_string_test.c

gtests/net/packetdrill/packetdrill.c

gtests/net/packetdrill/parse.h

gtests/net/packetdrill/run_system_call.c

gtests/net/packetdrill/script.c

gtests/net/packetdrill/types.h