Use arkindex-export to create the test fixture DB

The CLI's tests use two database fixtures: alto.sqlite, a database directly committed into the repo for arkindex export alto, and structure.sql, used by everything else, including (unnecessarily) arkindex export alto. This SQL file includes a database structure that we need to update manually, but we could instead use the arkindex export library to create the database structure, then fill it in with the SQL file. This avoids undetected bugs when we update the export's structure.