LUCENE-5207: Add a unused test method to make sure that if we change the FunctionValues interface we get compile error. Also make the class format version a constant for easy maintenance (once we backport)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5207@1522925 13f79535-47bb-0310-9956-ffa450edef68