Add a disabled unit test for bug #56812, based on code and file from Geoff Wright

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1615803 13f79535-47bb-0310-9956-ffa450edef68
Fix bug #56812 - In XSLF provide a way to get the URI of externally linked pictures

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1615812 13f79535-47bb-0310-9956-ffa450edef68