check-qjson: Fix utf8_string() to test all invalid sequences

check-qjson: Fix utf8_string() to test all invalid sequences

Some of utf8_string()'s test_cases[] contain multiple invalid
sequences.  Testing that qobject_from_json() fails only tests we
reject at least one invalid sequence.  That's incomplete.

Additionally test each non-space sequence in isolation.

This demonstrates that the JSON parser accepts invalid sequences
starting with \xC2..\xF4.  Add a FIXME comment.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-15-armbru@redhat.com>