Improve random sampling test

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1636821 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6038: Re-enable FieldValueFilter optimizations based on the way the docsWithField bits are implemented.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1636827 13f79535-47bb-0310-9956-ffa450edef68