move base test classes to oftest.base_tests

This lets them be shared between separate directories of tests.
diff --git a/tests/bsn_ipmask.py b/tests/bsn_ipmask.py
index ce785c9..70ca990 100644
--- a/tests/bsn_ipmask.py
+++ b/tests/bsn_ipmask.py
@@ -8,7 +8,7 @@
 import oftest.controller as controller
 import oftest.cstruct as ofp
 import oftest.message as message
-import basic
+import oftest.base_tests as base_tests
 
 from oftest.testutils import *
 
@@ -36,7 +36,7 @@
     else:
         return (1 << (63 - index)) - 1
 
-class BSNConfigIPMask(basic.SimpleDataPlane):
+class BSNConfigIPMask(base_tests.SimpleDataPlane):
     """
     Exercise BSN vendor extension for configuring IP source/dest match mask
     """