Don't use mock non-exist method assert_called_once

There isn't method assert_called_once of mock class
and hence does not really check anything. Let's use
assertEqual(1, mock.call_count) to check.

Change-Id: I2bdf62b775badc0adbe2c6f55bcc65cd13ac2d60
Merge "Don't use mock non-exist method assert_called_once"