In some corner cases, the `psutil.Process.children()` function might
throw a `NoSuchProcess` in case the parent died in the meantime. Wrap
the `.children()` function in a `_get_children` function to guard
against this case.
The `pid_has_signal` function can throw an exception if in the meantime
the program pid has disappeared (`FileNotFoundError`). Handle this
corner case.
Implement override mechanisms via command-line and/or env for the
following aspects:
* The maximum memory set by `test-harness`
* The timeout handled by `test-harness`
* The number of concurrent jobs to spawn while running `revng mass-testing run`
Giacomo Vercesi