Add a mockfs that hides the real absolute path of test temporary directories. [LUCENE-6443]

Any test that does processing based on full path names won't reproduce well, because checkout locations differ according to developer or jenkins machines, or even because the test gets assigned to a different slave JVM, etc, etc.

Instead we could add a mockfs that presents all files in the temp dir as having absolute paths of something predictable to improve reproducibility.

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Migrated from [LUCENE-6443](https://issues.apache.org/jira/browse/LUCENE-6443) by Robert Muir (@rmuir)

No files, tests, or entry points are named. Start by defining the mockfs scope and how test temporary-directory paths should be presented; done would mean files in the temporary directory expose predictable absolute paths so path-based processing is reproducible.

Written by the indexing model from the issue text.