Test that tenant list function can use auth_url

Listing tenants can be done on the auth URL because it is often needed
to be performed with an unscoped token. The behaviour is not tested
though so add a test to cover it.

Change-Id: Ic14d54f0f1d8cca2c3a5a6a6da7465c703773aa0
Merge "Test that tenant list function can use auth_url"