Test suite calls undefined save_pcap_file method in " the network device has its default MAC address configured"

Originally created by @intrigeri on #11698 (Redmine)

calling as root: . /usr/local/lib/tails-shell-library/hardware.sh && get_all_ethernet_nics
call returned: [0, "eth0\n", ""]
    Then 1 network interface is enabled                                                             # features/step_definitions/mac_spoofing.rb:75
calling as root: . /usr/local/lib/tails-shell-library/hardware.sh && get_all_ethernet_nics
call returned: [0, "eth0\n", ""]
calling as root: . /usr/local/lib/tails-shell-library/hardware.sh && get_all_ethernet_nics
call returned: [0, "eth0\n", ""]
calling as root: . /usr/local/lib/tails-shell-library/hardware.sh && get_current_mac_of_nic eth0
call returned: [0, "50:54:00:ce:30:6d\n", ""]
    And the network device has its default MAC address configured                                   # features/step_definitions/mac_spoofing.rb:11
      undefined local variable or method `save_pcap_file' for #<Object:0x000000091d2e10> (NameError)
      ./features/step_definitions/mac_spoofing.rb:28:in `/^the network device has (its default|a spoofed) MAC address configured$/'
      features/mac_spoofing.feature:19:in `And the network device has its default MAC address configured'
    And the real MAC address was leaked                                                             # features/step_definitions/mac_spoofing.rb:34
      Scenario failed at time 02:27:30

Perhaps we should instead call:

@sniffer.save_pcap_file

… as we do in other parts of the test suite?

Parent Task: #10288 (closed)