Adjust test to write the failing PNG in case expected and actual do not match

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568740 13f79535-47bb-0310-9956-ffa450edef68
Fix unit test which failed after new corrupted file was added.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568741 13f79535-47bb-0310-9956-ffa450edef68