tests: improve this test.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1580703 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5543: Add allowReadingFilesStillOpenForWrite to MockDirectoryFactory and MockDirectoryWrapper and set it to true for cloud tests.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1580707 13f79535-47bb-0310-9956-ffa450edef68