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.