LUCENE-4752: Preliminaries:
 - move useful assert*Equals from TestDuelingCodecs to LuceneTestCase,
 - rename sort to wrap in SortingAtomicReader to better suggest that the returned reader is a view.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1459037 13f79535-47bb-0310-9956-ffa450edef68