tools: pass constant to logger instead of string

On a few of our installations (namely CentOS), passing 'INFO'
resulted in a silent loglevel. Use a logging constant instead.

Cherry-picked from nodejs/node@8606793
Original commit metadata follows:
  Fixes: nodejs/build#104
  PR-URL: nodejs/node#1842
  Reviewed-By: Rod Vagg <[email protected]>

Reviewed-By: Julien Gilli <[email protected]>
PR-URL: nodejs#25653

tools/test.py

@@ -1375,7 +1375,7 @@ def Main():
 
   ch = logging.StreamHandler(sys.stdout)
   logger.addHandler(ch)
-  logger.setLevel('INFO')
+  logger.setLevel(logging.INFO)
   if options.logfile:
     fh = logging.FileHandler(options.logfile)
     logger.addHandler(fh)