Tests with pcap files

[dcferreira]

Tests with pcap files with multiple encapsulation layers (Ethernet, SLL) should be written, to ensure critical code doesn't break.

[dcferreira]

Some files that might be good for testing:

• scps_packet.pcap: contains a forged SCPS packet (that is, just the IP header with proto=105). The exporter should be able to extract protocolIdentifier == 105, even though gopacket doesn't support it (https://godoc.org/github.com/google/gopacket/layers#IPProtocol) --- scps_packet.tar.gz

• ospf_packet.pcap: contains a real OSPF packet --- ospf_packet.tar.gz