Use CRM_Core_BAO_SchemaHandler::createIndexes as a safe way to add indexes on upgrade

I want to add some indexes in a way that won't break 4.6 sites that have already added them. I found this function

CRM_Core_BAO_SchemaHandler::createIndexes

and added a test to check it works and (at least on a single field) it does so adding that test & switching some existing index-adds to use it to set the pattern