check-qjson: Consolidate partly redundant string tests

simple_string() and single_quote_string() have become redundant with
escaped_string(), except for embedded single and double quotes.
Replace them by a test that covers just that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-12-armbru@redhat.com>
1 file changed