* Add test for client getting correct model assertion and that the
returned model assertion matches the expected assertion parsed from
the raw response
* Add test for client getting correct serial assertion and that the
returned serial assertion matches the expected assertion parsed from
the raw response
* Add test that we get the expected error with a specific error response
for ErrNoState for model and serial endpoints.
Signed-off-by: Ian Johnson <ian.johnson@canonical.com>