test-qobject-input-visitor: Avoid format string ambiguity

test-qobject-input-visitor: Avoid format string ambiguity

When visitor_input_test_init_internal()'s argument @ap is null, then
@json_string is interpreted literally, else it's gets %-escapes
interpolated.  This is awkward.

One caller always passes null @ap, and the others never do.  Lift the
building of the QObject into the callers, where it can be done without
such ambiguity.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180806065344[email protected]>