Fix Lucene tests - check if the directory exists before trying to clean it up.

This fixes the previously failing tests for Zend_Search: https://travis-ci.org/zendframework/zf1/jobs/8413247#L923


https://travis-ci.org/mhujer/zf1/jobs/8414157#L916
Executing Zend/Search/AllTests.php
PHPUnit 3.4.15 by Sebastian Bergmann.
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