fix prefill test

Author: richvdh

tests/util/caches/test_descriptors.py

@@ -28,7 +28,6 @@
     current_context,
     make_deferred_yieldable,
 )
-from synapse.util.async_helpers import ObservableDeferred
 from synapse.util.caches import descriptors
 from synapse.util.caches.descriptors import cached
 
@@ -470,9 +469,9 @@ def func(self, key):
 
         a = A()
 
-        a.func.prefill(("foo",), ObservableDeferred(d))
+        a.func.prefill(("foo",), 456)
 
-        self.assertEquals(a.func("foo").result, d.result)
+        self.assertEquals(a.func("foo").result, 456)
         self.assertEquals(callcount[0], 0)
 
     @defer.inlineCallbacks