LUCENE-5969: add TestUtil.getDefaultCodec()

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1626781 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: improve this test, we cant uncomment the assert until we fix 5.0 codec

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1626796 13f79535-47bb-0310-9956-ffa450edef68