Adds new RT unit tests for _sync_compute_node

Adds new unit tests for the resource tracker's _sync_compute_node()
method. These new unit tests should make the objectification of the
resource tracker easier to do, since the old unit tests in
test_resource_tracker.py are brittle and hard to change.

Change-Id: I3e8d179d8c3856084fee595c8b0396a8a5db8aa4
Blueprint: make-resource-tracker-use-objects
Merge "Adds new RT unit tests for _sync_compute_node"