MAINT: Add nullctx back to test_utils.

Temporary upstream compat for Quantopian code.

To be removed at the earliest possible convenience.

zipline/utils/test_utils.py

@@ -7,6 +7,9 @@
 import operator
 from zipline.finance.blotter import ORDER_STATUS
 from zipline.utils import security_list
+# FIXME: This is downstream compat. If it's still here by August 5, 2015,
+# please yell at Scott Sanderson.
+from zipline.utils.control_flow import nullctx # noqa
 
 from six import (
  itervalues,