SOLR-5182: add regenerator for blockjoin cache

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1516653 13f79535-47bb-0310-9956-ffa450edef68
remove code duplication in tests

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1516661 13f79535-47bb-0310-9956-ffa450edef68