move testutils to src/python/oftest

testutils.py needs to be in a common location for different directories of
tests to use it.
diff --git a/tests/bsn_ipmask.py b/tests/bsn_ipmask.py
index cb64cb5..489b980 100644
--- a/tests/bsn_ipmask.py
+++ b/tests/bsn_ipmask.py
@@ -9,7 +9,7 @@
 import oftest.message as message
 import basic
 
-from testutils import *
+from oftest.testutils import *
 
 #@var port_map Local copy of the configuration map from OF port
 # numbers to OS interfaces