selftests: mptcp: capture pcap on both sides

When investigating performance issues that involve latency / loss /
reordering it is useful to have the pcap from the sender-side as it
allows to easier infer the state of the sender's congestion-control,
loss-recovery, etc.

Allow the selftests to capture a pcap on both sender and receiver so
that this information is not lost when reproducing.

This patch also improves the file names. Instead of:

  ns4-5ee79a56-X4O6gS-ns3-5ee79a56-X4O6gS-MPTCP-MPTCP-10.0.3.1.pcap

We now have something like for the same test:

  5ee79a56-X4O6gS-ns3-ns4-MPTCP-MPTCP-10.0.3.1-10030-connector.pcap
  5ee79a56-X4O6gS-ns3-ns4-MPTCP-MPTCP-10.0.3.1-10030-listener.pcap

It was a connection from ns3 to ns4, better to start with ns3 then. The
port is also added, easier to find the trace we want.

Co-developed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>