add MAGIC header so we will know if Lucene3xRW (from test-framework) ever tries to incorrectly read a non-Lucene3xRW file

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_10@1631340 13f79535-47bb-0310-9956-ffa450edef68