Test some Wasm instructions more strictly

[keithw]

Hi, thank you for SIMDe! I'm working on wasm2c and trying to get the full Wasm testsuite passing when using SIMDe as backend for the Wasm simd128 operations. We're seeing a small number of differences from the Wasm-specified behavior (WebAssembly/wabt#2021 (comment)), most involving either negative zero or NaNs.

As a first step, here is a PR that cranks up the strictness of the Wasm tests for demote, floor, max, min, nearest, pmax, pmin, and trunc. Currently these are tested to within the nearest 0.1; this PR would change that to an exact comparison (which I think is acceptable even in a fast-math situation as these are basically all selectors or integer truncation?). This also changes the overall test.h to do an exact bitwise comparison when slop=0, rather than ==.

The intention is to write tests that capture the Wasm behavior related to negative zero and implement these instructions in a conforming way. (E.g., Wasm requires that min(-0, 0) → -0 , and checking that requires a bitwise comparison or at least signbit().)

(Update: removed nearest from this list because it already has a test that involves negative zero. This test passes on GCC using __builtin_roundeven but fails on clang with SIMDe's C implementation of simde_math_roundeven.)

[mr-c]

Thanks!

[keithw]

Likewise -- thank you! We'll be really happy when we can check the box for SIMD support in wasm2c.