Story time:

In the days of yore, we generated PCAPs using tcpdump, which will
flush any buffered packets to its output file on reception of
SIGUSR2. Therefore we sent a USR2 before terminating the capture.

At some point, we switched over to use tshark to capture packets for
us, but the SIGUSR2 sayed in place.

It turns out, tshark will just die on USR2, abandoning the dumpcap
child it has created, which means the enclosing network namespace
can't be properly torn down.

Therefore:

Remove skip sending USR2 and just send TERM directly.

This is needed to create namespaces with MACVLANs from different
parent interfaces. The first consumer will be a Two-Port Mac
Relay (TPMR), so that we can control the flow of packets between two
nodes.

This mode let's us receive all packets from the parent device, which
is automatically set in promiscuous mode, and let's the MACVLAN
transmit packets with any SA, which we need to implement a TPMR (or a
bridge or anything else we might dream up)

This gets rid of annoying "Address already in use" errors when running
tests repeatedly from a test-sh.

When used as a context manager, __exit__ will be called even though
__enter__ might throw an exception half way through. So there is no
guarantee that the instance is on the cleanup list when we get to
stop().

Now that we're using the `passthru` mode of MACVLANs, it seems we can
rely on the parent interface's promiscuity going down to 0 as an
indication that the kernel has completed the removal of the interface,
and will accept the creation of a new one.

This is a specialized namespace, containing two ports,
which (hopefully) acts as completely transparent "bump-on-the-wire"
bridge.

Useful in scenarios where you want to test what happens when the flow
of packets between two nodes is disrupted. E.g., for testing fail-over
behavior in dynamic routing protocols, L2 redundancy, VRRP etc.

Initially done as a workaround for the MACVLAN create/remove race. But
even though the old version of this test now works, this refactor
provides better test step descriptions, making it easier to follow for
anyone looking to manually verify the functionality.

[skip-ci]