test-qobject-input-visitor: Abort earlier on bad test input

visitor_input_test_init_internal() parses test input with
qobject_from_jsonv(), and asserts it succeeds.  Pass &error_abort for
good measure.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1488317230-26248-12-git-send-email-armbru@redhat.com>