DERBY-6505: Clean up dead code in FileUtil

Add test that exercises FileUtil methods that are only used in soft
upgrade from 10.8 or earlier when backing up databases that contain
jar files.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1576691 13f79535-47bb-0310-9956-ffa450edef68