LUCENE-5708: remove IWC.clone

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1598489 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5708: fix test bug

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1598497 13f79535-47bb-0310-9956-ffa450edef68
listFiles can return null if a different thread removes the dir concurrently (may happen in Solr tests),..

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1598499 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5708: fix test bug

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1598503 13f79535-47bb-0310-9956-ffa450edef68