Add functional test for device_exists() with invalid namespace

This should return False, and not throw an exception because
the namespace doesn't exist.

Change-Id: I5f787b12abc156452bceed7fae43598980a3a93c

neutron/tests/functional/agent/linux/test_ip_lib.py

@@ -103,6 +103,9 @@ class IpLibTestCase(IpLibTestFramework):
         self.assertTrue(
             ip_lib.device_exists(device.name, namespace=attr.namespace))
 
+        self.assertFalse(
+            ip_lib.device_exists(attr.name, namespace='wrong_namespace'))
+
         device.link.delete()
 
         self.assertFalse(