Fix transcripts in XML course when no static_asset_path is set.

Case where static_asset_path is not set in XML course.

In such case use data_dir.
Issue: https://openedx.atlassian.net/browse/BLD-67

@cpennington @carsongee 
I would prefer the test name to have xml in it as it was before, maybe `test_translation_static_transcript_xml_with_data_dir` if you want to keep the data_dir in there, but up to you if you prefer this.