check-qjson: More thorough testing of UTF-8 in strings

Test cases are scraped from Markus Kuhn's UTF-8 decoder capability and
stress test at
http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt

Unfortunately, both JSON parser and formatter misbehave right now.
This test expects current, incorrect results.  They're all clearly
marked, and are to be replaced by correct ones as the bugs get fixed.
See comments in new utf8_string() for details.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
