I've tried many times to upload theme-unit-test-data.xml from http://codex.wordpress.org/Theme_Unit_Test But when I'm trying to upload this file in WordPress Importer it gives me an error saying:
Sorry, there has been an error. This does not appear to be a WXR file, missing/invalid WXR version number
I'm using the latest version (4.0) of the WordPress CMS.
Edit: I reinstalled the WordPress importer plugin and downloaded the Theme Unit test XML file multiple times with the same result.