Increase the font-inaccuracy for one unit test as some machines have fonts that lead to lower values, e.g. we get only 770 instead of 980 for Jenkins build on IBM JDK and on some local Windows machines

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1597873 13f79535-47bb-0310-9956-ffa450edef68