Add unit test

tests/util/test_stream_change_cache.py

@@ -178,6 +178,22 @@ class StreamChangeCacheTests(unittest.TestCase):
             ),
         )
 
+        # Query a subset of the entries mid-way through the stream. We should
+        # only get back the subset.
+        self.assertEqual(
+            cache.get_entities_changed(
+                [
+                    "bar@baz.net",
+                ],
+                stream_pos=2,
+            ),
+            set(
+                [
+                    "bar@baz.net",
+                ]
+            ),
+        )
+
     def test_max_pos(self):
         """
         StreamChangeCache.get_max_pos_of_last_change will return the most