Fix method mocked in a test

Mock `api_client.alarms.list` instead of `api_client.alarms` in the
tests/alarm/partition/test_coordination.py:TestCoordinate.test_assigned_alarms
method.

Change-Id: Id33f9854de4604b47b2a453c91a58f2dcd15e682
Closes-Bug: #1329701
Merge "Fix method mocked in a test"