LUCENE-2598: factor the behavior of MockRAMDirectory into MockDirectoryWrapper, add experimental -Dtests.directory= to allow running the tests under different directory impls [but the default is still RAMDirectory]
LUCENE-2598: factor the behavior of MockRAMDirectory into MockDirectoryWrapper, add experimental -Dtests.directory= to allow running the tests under different directory impls [but the default is still RAMDirectory]

git-svn-id: 308d55f399f3bd9aa0560a10e81a003040006c48@988206 13f79535-47bb-0310-9956-ffa450edef68

cscorley/solr-only-mirror,cscorley/solr-only-mirror,cscorley/solr-only-mirror