Changes

tests/{ctrl,vty}_test_runner.py: dump stdout/stderr

From time to time, we're seeing the following error in Jenkins:

----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../tests/vty_test_runner.py", line 70, in tearDown
    raise Exception("Process returned %d" % rc)
Exception: Process returned -9
----------------------------------------------------------------------

Let's dump stdout/stderr of the process to get more information.

Change-Id: Ie6a2f5b1bc56d35513643b52923403798e5a0a5a
Related: OS#5665