Test: Automation for netsniff-ng packet capture

[MateuszGrabuszynski]

Adds automation capabilities for netsniff-ng packet capture instead of tcpdump. As both should not be executed at once, such option was not added.

• in tests/validation/configs/test_config.yaml added tool in .capture_cfg with following options recognized: tcpdump for tcpdump, netsniff or netsniff-ng for netsniff-ng
• in tests/validation/create_pcap_file/netsniff.py created NetsniffRecorder class based on TcpDumpRecorder that can serve as netsniff-ng capturing process class
• added conditional execution of netsniff-ng packet capture in
  • tests/validation/mtl_engine/GstreamerApp.py,
  • tests/validation/mtl_engine/ffmpeg_app.py
  • and tests/validation/mtl_engine/udp_app.py
• modified comments about configuration collection and modification so they reflect actual situation

TODO:

• Prepare installation automation for seamless execution
• Eliminate duplication by creating a parent Recorder class interface for capture
• Think if the capture can be done via a fixture with automated (code-less*) test_name generation
• Check whether the interface name is always produced properly in the command and fix if needed, then remove FIXME comment

*code-less here means without interfering with the test code