chore: [spv-in] atomics snapshot tests

[schell]

Connections

• [spv-in] Atomics support #4489
• [naga spv-in] Atomic tests should use the standard snapshot machinery #6292

Description
Moves the atomic operations support tests out of the SPIR-V frontend and into the standard snapshot tests.

Fixes #6292.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[jimblandy]

Unless there's a particular reason we're concerned that the SPIR-V front end might/needs to produce interesting output on particular backends, we generally only produce WGSL output.

The idea of the snapshots is that we do not actually convert every input file to every output language. Rather, we keep the "wings of the butterfly" separate:

• We test all the front ends by converting all input languages to WGSL.
• We test all the backends by converting WGSL inputs to every output language.

So unless there is special code in the backends that is only exercised by SPIR-V input (I don't think that's the case, but I'm not totally positive??), please only produce WGSL output for these input files.

[schell]

I've removed the extraneous snapshot output files 👍 .