SOLR-4952: solrconfig.snippet.randomindexconfig.xml used in more configs, including eliminating TestPluginEnable and folding it into TestConfig (merge r1513691)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1513698 13f79535-47bb-0310-9956-ffa450edef68