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>
diff --git a/test/test_ip4.py b/test/test_ip4.py
index b13a304..b6553db 100644
--- a/test/test_ip4.py
+++ b/test/test_ip4.py
@@ -395,7 +395,7 @@
 
         for ip in ips:
             self.assertTrue(_ip_in_route_dump(ip, fib_dump),
-                            'IP {} is not in fib dump.'.format(ip))
+                            'IP {!s} is not in fib dump.'.format(ip))
 
     def verify_not_in_route_dump(self, fib_dump, ips):
 
@@ -406,7 +406,7 @@
 
         for ip in ips:
             self.assertFalse(_ip_in_route_dump(ip, fib_dump),
-                             'IP {} is in fib dump.'.format(ip))
+                             'IP {!s} is in fib dump.'.format(ip))
 
     @classmethod
     def setUpClass(cls):