test: improve testing next items

tests/test_scheduling_domain.py

@@ -251,6 +251,9 @@ class TestDomain(unittest.TestCase):
         self.assertEqual(2, len(next))
         next = e3.get_all_next(False)
         self.assertEqual(3, len(next))
+        self.assertEqual(e4.source, next[0].source)
+        self.assertEqual(e5.source, next[1].source)
+        self.assertEqual(e6.source, next[2].source)
 
     def test_playlist_item_start_end(self):
         print(self._testMethodName)