GEOMESA-202 Consolidate unit tests so they share one AccumuloDataStore

Found areas where Mock Accumulo instances could be shared between tests,
merged some of the separate Iterator tests into the MultiIteratorTest,
added @jnh5y's indexSchemaFormat suggestion to most test DataStore params,
and cleaned up some specs2 code style. All changes should save about
30 secs in complete build time.
it may be better to use the new index builder that @elahrvivaz wrote here