Disabled autoRefresh when inserting fixtures

refs https://github.com/TryGhost/Toolbox/issues/592

- we don't need to receive the refreshed model back afterwards so we can
  save an SQL select per insert by disabling auto-refresh
- this saves about 18000 DB queries for the MySQL tests, which should
  help reduce test time due to the overhead

ghost/core/core/server/data/schema/fixtures/FixtureManager.js

@@ -69,6 +69,7 @@ class FixtureManager {
      */
     async addAllFixtures(options) {
         const localOptions = _.merge({
+            autoRefresh: false,
             context: {internal: true},
             migrating: true
         }, options);