Randomize lockfactories in tests [LUCENE-6509]

Today we don't do this, and it can be annoying for tests (see LUCENE-6507).

But we should randomize between NIO/Simple. LuceneTestCase can "remember" which ones have been assigned and prevent the issue of opening IW with NIO and then making a new dir over the same path with simple and confusing the test.

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Migrated from [LUCENE-6509](https://issues.apache.org/jira/browse/LUCENE-6509) by Robert Muir (@rmuir)

Start by reading LuceneTestCase and the existing NIO/Simple lock factory selection, then review LUCENE-6507 for the failure mode. Done means tests randomize between NIO and Simple while preventing conflicting lock-factory assignments for the same path.

Written by the indexing model from the issue text.