test_cert_plugin: Encode 'certificate' for comparison with 'usercertificate'
The 'certificate' option is Str, but 'usercertificate' is Bytes.
Decode before comparing one with the other.
Part of the work for: https://fedorahosted.org/freeipa/ticket/4985
Reviewed-By: Martin Basti <mbasti@redhat.com>