[migrations/tests] sort results before assertion (#23347)

There have been several failures in this test, seemingly caused by a lack of sorting in the results. It makes sense that since both migrations are run simultaneously that sometimes one would succeed and sometimes another would, so I've just sorted the results before checking.

![image](https://user-images.githubusercontent.com/1329312/45791153-44e9cc80-bc3d-11e8-88c4-760d4c7b35bd.png)

cc: @chrisdavies

test/api_integration/apis/saved_objects/migrations.js

@@ -181,7 +181,7 @@ export default ({ getService }) => {
       // The polling instance and the migrating instance should both
       // return a similar migraiton result.
       assert.deepEqual(
-        result.map(({ status, destIndex }) => ({ status, destIndex })),
+        result.map(({ status, destIndex }) => ({ status, destIndex })).sort((a) => a.destIndex ? 0 : 1),
         [
           { status: 'migrated', destIndex: '.migration-c_2' },
           { status: 'skipped', destIndex: undefined },