wrap scapy with explicit imports

pylint can't handle scapy's magical imports.
diff --git a/tests/pktact.py b/tests/pktact.py
index f803cfd..04edf95 100644
--- a/tests/pktact.py
+++ b/tests/pktact.py
@@ -17,7 +17,7 @@
 import time
 import unittest
 import random
-import scapy.all as scapy
+import oftest.packet as scapy
 
 from oftest import config
 import oftest.controller as controller