need a low-level base directory assert class [LUCENE-3383]

idea is to have some base helper class in test-framework
that tests directories, so that we get better coverage out of anything sitting in contrib,
and out of our existing directories (not relying upon random testing to eventually catch stuff, and
not relying upon directories being in core lucene to have good tests).

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Migrated from [LUCENE-3383](https://issues.apache.org/jira/browse/LUCENE-3383) by Robert Muir (@rmuir)

Start by reading the test-framework and existing directory tests, then inspect directory implementations in contrib and core Lucene to identify the common behaviors that need coverage. Done means defining and integrating a reusable base helper class that exercises those directory behaviors rather than relying on random testing.

Written by the indexing model from the issue text.