Add unit test for compute_node_tracker

[hongbin]

The module compute_node_tracker (https://github.com/openstack/zun/blob/master/zun/compute/compute_node_tracker.py) doesn't have unit test coverage. We need to add unit tests for that. It will help to capture issues like: https://bugs.launchpad.net/zun/+bug/1753632 .

The compute_node_tracker is similar to the resource_tracker in nova, so we can use test_resource_tracker (https://github.com/openstack/nova/blob/master/nova/tests/unit/compute/test_resource_tracker.py) as an example for writing test cases.