Abstract class to do basic tests for a docvalues format. NOTE: this test focuses on the docvalues impl, nothing else. The [stretch] goal is for this test to be so thorough in testing a new DocValuesFormat that if this test passes, then all Lucene/Solr tests should also pass. Ie, if there is some bug in a given DocValuesFormat that this test fails to catch then this test needs to be improved!