Major Description
See http://jenkins.thetaphi.de/job/Lucene-Solr-master-MacOSX/3349
The bug here is that we do this crazy copyFrom of all the index files without fsyncing it. So MockDirectoryWrapper treats it as corrupted.
But while investigating I noticed that RAMdir is incredibly lenient, will happily truncate existing files and so on. I think we should fix that here too.