json: enhance parser test
    
    We already enable the parser option to detect invalid UTF-8, but
    didn't test it.  Also, JSON states that behavior of an object
    with a duplicated key is undefined; we chose to reject it, but
    were not testing it.
    
    With the enhanced tests in place, we can simplify yajl2
    initialization by relying on parser defaults being sane.
    
    * src/util/virjson.c (virJSONValueFromString): Simplify.
    * tests/jsontest.c (mymain): Test more bad usage.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>