Also scan stderr for audio files to test, due to change in Android test_runner

BUG=chromium:733108
NOTRY=True

Review-Url: https://codereview.webrtc.org/2935263002
Cr-Commit-Position: refs/heads/master@{#18595}

webrtc/audio/test/low_bandwidth_audio_test.py

@@ -198,7 +198,7 @@ def main():
     test_process = subprocess.Popen(
         _LogCommand(test_command + ['--sample_rate_hz=%d' %
                                     analyzer.sample_rate_hz]),
-        stdout=subprocess.PIPE)
+        stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     try:
       lines = iter(test_process.stdout.readline, '')
       for result in ExtractTestRuns(lines, echo=True):