Add zombie tracking to test suite

[goodboy]

Been doing this manually for a while but we need an autouse fixture which just always checks this and fails the suite on a leak.

Though this is likely not an issue after the work in #245, work in the zombie_lord_infinite branch (as is) will need this in order to get clean CI runs and zero zombies (that branch is very WIP nursery / supervisor rework).

todo:

• psutil can do it easily with following snippet:

import psutil
 for proc in psutil.process_iter():
     line = proc.cmdline()
     name = proc.name()
     if (
         'tractor._child' in line
     ):
         print(line)

• this should tie in with We aint got zombie shields #188 and Hard kill tests #145
• what should be the failure mode when we leak a zombie?
• required before we move forward on https://github.com/goodboy/tractor/tree/zombie_lord_infinite