test-fix-xml-importer: Fixes unit test failure in XML importer

Backported from upstream:
https://github.com/edx/edx-platform/commit/d7cf8b7fa0f2880e113145122b167d9e34c3421d#diff-19
Merge pull request #234 from edx-solutions/test-fix-xml-importer

test-fix-xml-importer: Fixes unit test failure in XML importer