VPP-1508: Python3 tests. Explicitly specify string formatting.

.format() doesn't understand what to do with binary values by default.
Specify that we expect coersion to string.

Change-Id: Ic4b697f70852124dc85ab231f76b7934d0d71f4d
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/log.py

@@ -57,7 +57,7 @@ def get_logger(name):
 
 
 def get_parallel_logger(stream):
-    logger = logging.getLogger('parallel_logger_{}'.format(stream))
+    logger = logging.getLogger('parallel_logger_{!s}'.format(stream))
     logger.propagate = False
     logger.setLevel(logging.DEBUG)
     handler = logging.StreamHandler(stream)