arb_texture_view: allow a few alternative values

arb_texture_view: allow a few alternative values

This test incorrectly assumes "perfect" precision when converting unorm
values to float. Extending this test to perfectly handle every legal
value would be a big and complicated change.

But in reality, there's really only two sets of values that are
"reasonable" here, values computed using "x / 255.0f" or values computed
using "x * (1.0f / 255)". There's no obvious way of generating different
values here, so let's just "white-list" both variants.

This means we'll have to pass two expected values, one with each
equation. To keep things simple, we assume that a single implementation
only ever generates values of one of the two kinds at once.

This isn't perfect, but allows us to keep an optimization in LLVMpipe
while passing these tests.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/537>