test: change duration_ms to duration

The test script (tools/test.py) logs duration as "duration_ms: x.y".
This is confusing (as the duration is measured in seconds).

New example output: duration: 0.212s

PR-URL: nodejs#7133
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

tools/test.py

@@ -310,7 +310,7 @@ def HasRun(self, output):
       (duration.seconds + duration.days * 24 * 3600) * 10**6) / 10**6
 
     logger.info('  ---')
-    logger.info('  duration_ms: %d.%d' % (total_seconds, duration.microseconds / 1000))
+    logger.info('  duration: %d.%ds' % (total_seconds, duration.microseconds / 1000))
     logger.info('  ...')
 
   def Done(self):