Updated PCAP plugin to (again) use updated model names

Changed old names (like Layer3Interface and layer3_interface) to be the
updated names (like L3If and l3_if).
Renamed PCAP test file name to make it consistent with plugin name

Went from pcap_parser_test.rb to pcap_test.rb since the plugin name is
pcap.
Test for PacketFu (and OUI parser, and Modbus PacketFu additions)

This is essentially the test that used to be called PcapParserTest.
Added tests specific to `pcap` plugin