Fix test_get_resource_classes parameter passing
Fix test_get_resource_classes parameter passing

In utils.get_resource_classes we expect a list of strings.
However we are passing a string.
The test was succeding by pure luck.
If we pass something else (such as 'Stacks') we had multiple results.
This test was confusing for other developers implementing other tests.

Closes-Bug: #1820033

Change-Id: I8414b7582d94b199457a01e4916a7e7b6e51edbc

openstack/ospurge,openstack/ospurge