1171509 - FastForwardXmlFileContext was sometimes using the wrong starting file

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1171509
You could use mock on mtime to do this rather than sleeping. It would also be nice to not have the unit tests interact with the filesystem (not that this is the greatest of our current testing concerns, but it would be a good long term goal.)