tests/string-input-visitor: Add negative integer tests

Add two negative tests, one for int and one for int16List.  The latter
exposes a bug: nonsensical input results in an empty list instead of
an error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1461325048-14122-1-git-send-email-armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-22-git-send-email-eblake@redhat.com>
