extra/virtual-stb: Add unit test for netstat output parsing

This test doesn't run the real netstat so it doesn't prove that the
function works in the real world, but at least it documents the parsing
code and what the expected input looks like.
extra/virtual-stb: Ignore blank lines in netstat output

This fixes an error introduced by the refactoring in my previous commit,
where `pid, name = line[...].split('/')` raised "ValueError: need more
than 1 value to unpack".