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

[asfimport]

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 by Robert Muir (@rmuir)