Fix tests with bodhi-client 6+
    
    bodhi-client was ported to use OIDC Client for authentication.
    We need to mock out its OIDC provider metadata discovery because
    it does an unavoidable network request, and we also need to make
    sure `HOME` is defined in `os.environ` because the new OIDC code
    expects to read it (in `BodhiClient._build_oidc_client()`).
    
    Merges: https://pagure.io/fedpkg/pull-request/482
    Signed-off-by: Adam Williamson <awilliam@redhat.com>