Fixed unit test TestRealNotification

The way how the unit test
test_notification.TestRealNotification.test_notification_service was
constructed would sometimes let the main test case check the result even
before all the expected notification listeners finish processing. This
would cause failure in gate test.

We should make sure the test case check the result after all the
expected listeners finish processing the notification.

Change-Id: I5333d83365fc39264b1002030494556f90319312
Closes-Bug: #1327344
Merge "Fixed unit test TestRealNotification"