Benchmark script: use process groups to manage children

[smowton]

Previously when a process timed out only the managing shell would be killed;
the underlying process would continue, starving later jobs of CPU time

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0a62867).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/102491205

[tautschnig]

Could we instead step-by-step Python-ize this code? Just completely doing the timed execution in Python:

import subprocess
import threading
import time

def run_cmd_with_timeout(cmd, timeout_sec):
    """
    Execute `cmd` in a subprocess and enforce timeout `timeout_sec` seconds.
    Return subprocess exit code on natural completion of the subprocess.
    Raise an exception if timeout expires before subprocess completes.
    """
    t1 = time.time()
    proc = subprocess.Popen(cmd)
    timer = threading.Timer(timeout_sec, proc.kill)
    try:
        timer.start()
        proc.communicate()
    finally:
        timer.cancel()
    t2 = time.time()
    return (proc.returncode, t2 - t1)

[smowton]

Fine with me -- if you want to make a PR to replace this whole script I'd vote for that one over this one!

[smowton]

If I can get a code owner review I'll merge this for now pending a rewrite