Improve process dead/alive detection for tests

[Flood1993]

Depends on #59

Perhaps we could use https://github.com/giampaolo/psutil

import psutil
psutil.pids()

[1, 2, 3, 4, 5, 6, 7, 46, 48, 50, 51, 178, 182, 222, 223, 224,
268, 1215, 1216, 1220, 1221, 1243, 1244, 1301, 1601, 2237, 2355,
2637, 2774, 3932, 4176, 4177, 4185, 4187, 4189, 4225, 4243, 4245, 
4263, 4282, 4306, 4311, 4312, 4313, 4314, 4337, 4339, 4357, 4358, 
4363, 4383, 4395, 4408, 4433, 4443, 4445, 4446, 5167, 5234, 5235, 
5252, 5318, 5424, 5644, 6987, 7054, 7055, 7071]

[Flood1993]

Process are remaining as zombie processes in the process table, so we cannot simply check whether they have an entry there. We must check the status to see whether it ended, and psutil comes in really handy for that.

[Flood1993]

In relation to this: http://stackoverflow.com/a/38775811/3021357

The double fork idea is interesting. I will do some research about it and see how it could be implemented.