Updated: unit tests, more variable random delay
Updated: unit tests now run workers in a separate process group
Updated: example_worker now kills all his children when it get killed

Dramatic story...
Fixed: remove worker's signal handling

...but prevent child process to go up in the stack
Fixed: tests: simply wait before touching the file to avoid some errors

The tests was failing because the delay of the example worker was
locking the row in the queue despite the touch file is created.
Therefore the count was sometimes bad.