tests: Have worker return immediately on bad executable

tests: Have worker return immediately on bad executable

No reason to waste CI cycles if we know the test will not run.

See:
17:55:11  ==============================================================================
17:55:11  VPP Object Model Test
17:55:11  ==============================================================================
17:55:11  Exception in thread Thread-4:
17:55:11  Traceback (most recent call last):
17:55:11    File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
17:55:11      self.run()
17:55:11    File "/w/workspace/vpp-beta-verify-master-ubuntu1804/test/framework.py", line 1475, in run
17:55:11      stdout=subprocess.PIPE, stderr=subprocess.PIPE)
17:55:11    File "/w/workspace/vpp-beta-verify-master-ubuntu1804/test/run/venv/local/lib/python2.7/site-packages/subprocess32.py", line 617, in __init__
17:55:11      restore_signals, start_new_session)
17:55:11    File "/w/workspace/vpp-beta-verify-master-ubuntu1804/test/run/venv/local/lib/python2.7/site-packages/subprocess32.py", line 1415, in _execute_child
17:55:11      raise child_exception_type(errno_num, err_msg)
17:55:11  OSError: [Errno 2] No such file or directory: '/w/workspace/vpp-beta-verify-master-ubuntu1804/test/build/vom_test/vom_test'
17:55:11
17:55:11  17:55:11,328 Timeout! Worker did not finish in 120s
17:55:11  run C++ VOM tests                                                        ERROR [ temp dir used by test case: /tmp/vpp-unittest-VOMTestCase-vpMcWF ]
17:55:11

Type: fix

Change-Id: I3d8252807e98a09a8abd70de8a22517151f9d786
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>