Add tests for XML export v0.10

The XML unit tests use XML data files using export v0.3.
MediaWiki export format is currently version v0.10.

Moved XML export v0.3 data files into tests/data/xml subdirectory.
Add allowed_failure test for edit summary not being decoded.

Bug: T85334
Change-Id: I82fccdafcf302d77dd3c6c5ce7713e72a4412e5c