Fix branch coverage for un-called callbacks. (#6242)

Use 'mock.Mock()' w/ 'return_value' True, rather than inlining
'lambda _: True' in cases where the callback doesn't get called.

api_core/tests/unit/test_bidi.py

@@ -280,7 +280,9 @@ def cancel(self):
 
 class TestResumableBidiRpc(object):
     def test_initial_state(self):
-        bidi_rpc = bidi.ResumableBidiRpc(None, lambda _: True)
+        callback = mock.Mock()
+        callback.return_value = True
+        bidi_rpc = bidi.ResumableBidiRpc(None, callback)
 
         assert bidi_rpc.is_active is False
 
@@ -380,7 +382,9 @@ def test_recv_recover_already_recovered(self):
             grpc.StreamStreamMultiCallable,
             instance=True,
             side_effect=[call_1, call_2])
-        bidi_rpc = bidi.ResumableBidiRpc(start_rpc, lambda _: True)
+        callback = mock.Mock()
+        callback.return_value = True
+        bidi_rpc = bidi.ResumableBidiRpc(start_rpc, callback)
 
         bidi_rpc.open()