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/serial_failover.py b/tests/serial_failover.py
index d12040e..311dde1 100644
--- a/tests/serial_failover.py
+++ b/tests/serial_failover.py
@@ -17,7 +17,7 @@
 import oftest.dataplane as dataplane
 import oftest.action as action
 
-from testutils import *
+from oftest.testutils import *
 
 #@var serial_failover_port_map Local copy of the configuration map from OF port
 # numbers to OS interfaces