Tests: Raise exception, don't raise string.

This was deprecated ~ python 2.4, and causes a TypeError as sideEffect.

>>> raise "foo"
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: exceptions must derive from BaseException

Change-Id: I4117b6d60ae896eaa1ef2a73a323d8d241f8c3a7
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/test_vxlan.py

@@ -111,7 +111,7 @@ class TestVxlan(BridgeDomain, VppTestCase):
                                               mcast_sw_if_index=1,
                                               is_add=is_add, vni=vni)
             if r.sw_if_index == 0xffffffff:
-                raise "bad sw_if_index"
+                raise ValueError("bad sw_if_index: ~0")
 
     @classmethod
     def add_shared_mcast_dst_load(cls):