Make test_export_all_courses not flaky

Current implementation of the test relies on the exact order in which `xml_importer.import_static_content` would return files in `static` dir. This function uses `os.walk` for retrieving list of the files and `os.walk` does not guarantee order of files retrieved, which might make test fail for no apparent reason.

review: @andy-armstrong @cahrens @auraz  @zubair-arbi