tests: make lack of json module a hard failure
tests: make lack of json module a hard failure

This module is available since python 2.6, removing this improves test
coverage.

Signed-off-by: Yann Dirson <9a3266eb84fe75b6e4a0d9d8e42ed70b568ec175@vates.fr>

xenserver/python-libs,xenserver/python-libs