Fix vararg function in wxXml unit test broken by recent changes.

Fix vararg function in wxXml unit test broken by recent changes.

The changes in r67345 changed CheckXml() vararg function to take a reference
as the first argument but this doesn't work with va_start(), so revert to
using a pointer here.

This fixes the current unit test failures in the XML tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67408 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775