Javadoc has some references to unit test classes, so put the junit jars on the javadoc classpath

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1798324 13f79535-47bb-0310-9956-ffa450edef68

build.xml

@@ -438,6 +438,7 @@ under the License.
         <path refid="scratchpad.classpath"/>
         <path refid="ooxml.classpath"/>
         <path refid="ooxml.xmlsec.classpath"/>
+        <path refid="test.jar.classpath"/>
         <path path="${env.CLASSPATH}"/>
     </path>