SolrTestCase.clearIndex should ensure IndexWriter.deleteAll is called

Normal deleteByQuery commands are subject to version constraint checks due to the possibility of out of order updates, but DUH2 has special support (triggered by version=-Long.MAX_VALUE for use by tests to override these version constraints and do a low level IndexWriter.deleteAll() call. A handful of tests override SolrTestCaseJ4.clearIndex() to take advantage of this (using copy/pasted impls), but given the intended purpose/usage of SolrTestCaseJ4.clearIndex(), it seems like the the base method in SolrTestCaseJ4 should itself trigger this low level deletion, so tests get this behavior automatically.