skip test if consensus file not found

This isn't a "failed" test per se, maybe stem should skip it?

======================================================================
                 test.integ.descriptor.networkstatus                  
======================================================================

test_cached_consensus                                        [SUCCESS]
test_cached_microdesc_consensus                              [FAILURE]
test_consensus_v2                                            [SUCCESS]
test_consensus_v3                                            [SUCCESS]
test_metrics_consensus                                       [SUCCESS]
test_metrics_vote                                            [SUCCESS]
test_vote                                                    [SUCCESS]

======================================================================
ERROR: test_cached_microdesc_consensus
----------------------------------------------------------------------
Traceback:
  File "/home/gsathya/stem/test/integ/descriptor/networkstatus.py", line 76, in test_cached_microdesc_consensus
    with open(consensus_path) as descriptor_file:
IOError: [Errno 2] No such file or directory: '/home/gsathya/stem/test/data/cached-microdesc-consensus'

----------------------------------------------------------------------
Ran 7 tests in 0.142s

FAILED