Avoid Python error in case of timeout

[matttbe]

When a test is blocked and after a timeout, this Python error is produced:

  Exception in thread Thread-1:
  Traceback (most recent call last):
    File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
      self.run()
    File "packetdrill/run_all.py", line 169, in run
      self.testset.RunDir(self.path)
    File "packetdrill/run_all.py", line 147, in RunDir
      self.PollTestSet(procs, time_start)
    File "packetdrill/run_all.py", line 136, in PollTestSet
      for proc, path, variant in procs:
  ValueError: too many values to unpack (expected 3)

That's simply because procs variable is an array of tuple containing 5 items. Two additional items are missing: stdout and stderr.

With the patch, we have a clearer error:

  KILL [/opt/packetdrill/gtests/net/mptcp/mp_join/mp_join_client.pkt (ipv4)]

A second commit prints more info in case of timeout if we are in verbose with log_on_error option.

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[matttbe]

@googlebot I signed it!

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[matttbe]

Code looks great, thanks. Please see the bot report on needing a CLA sign off.

@wdebruij thank you for the quick review! My administration work is now done :)

[nealcardwell]

Merged. Thanks, Matthieu!