fixtures/listen-on-socket-and-exit needs improved cleanup

While reviewing some of our buildbots, I've found some dead processes which all seems related to failing this test. We need to make cleanup more robust.

Just reminding myself that this still seems to be an issue. Found a stray process on one of the smartos machines that's been running from July 2nd (roughly one week). Edit: and one on a freebsd machine.

test: handling failure cases properly