check-qjson: Cover UTF-8 in single quoted strings

utf8_string() tests only double quoted strings.  Cover single quoted
strings, too: store the strings to test without quotes, then wrap them
in either kind of quote.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-13-armbru@redhat.com>