Fix test_secret_stores tests

There were five minor issues with the test_secret_stores tests:

1) There is a typo in some test cases. They are calling
   unset_peferred_secret_store instead of unset_p<r>eferred_secret_store

2) Set and unset preferred secret stores API calls in the
   SecretStoresClient expect 200 response status code when in fact they
   should expect 204 instead [1].

3) test_get_preferred_secret_store test expects to get preferred
   secret store when in fact none is set for the project.

4) skip_checks() function did not call super's skip_checks()

5) test_set_unset_preferred_secret_store test expects to get preferred
   secret store for a project when there is no preferred secret store
   set for it.

[1] https://docs.openstack.org/barbican/rocky/api/reference/store_backends.html#post-v1-secret-stores-secret-store-id-preferred

Change-Id: Ic211ea87006662c5a24aef3d1b78a5aa85b5e35b
2 files changed