If we don't the whole testing process will become killed when wcfs
becomes taught to kill clients that do not handle pin notifications
well.

Use multiprocessing to do so and to be able to interoperate with spawned
test process by sending/receiving objects to/from it.

Preliminary history:

    aef0f0e1Co-authored-by: Levin Zimmermann <levin.zimmermann@nexedi.com>