This is simply unnecessary and wastes time.
As part of this, simply remove the "all" directive. Only for a couple of tests
is it even potentially interesting to validate all pixels (e.g.
nointerpolation.shader_test), and for those "all" is replaced with an explicit
(0, 0, 640, 480) rect.
In all other cases we just probe (0, 0).
It is hard to initialize default values on add_assignment() and calling
add_assignment() for initializers is not really necessary: the only
thing we need from it the implicit cast.
