tenant_absolute_limits command is not being tested

We don't have a unit test to cover tenant_absolute_limits in the manila_api module. We need to add one for it. The gotcha for this is that we need to mock a limit object in order to be able to perform the test. We have fakes, but we use them for dashboard tests. It might be a good idea to refactor the existing code a bit to have fakes for api tests as well and be able to add the coverage for this specific tests in a clean way.

The code for the functionality that is missing the test can be seen in [0]. Our fakes live in [1].

[0] https://github.com/openstack/manila-ui/blob/master/manila_ui/api/manila.py#L473-L482
[1] https://github.com/openstack/manila-ui/blob/master/manila_ui/tests/dashboards/project/test_data.py