DERBY-6233: XMLBindingTest fails on latest JDK 8 EA

Set the system property javax.xml.accessExternalDTD in the test to
allow the XML parser to access the DTD.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1488116 13f79535-47bb-0310-9956-ffa450edef68