[naga spv-in] Atomic tests should use the standard snapshot machinery

[jimblandy]

At the moment, the SPIR-V front end's parsing of atomic operations is tested by custom tests in the test_atomic module in naga/src/front/spv/mod.rs. They should just use the standard snapshot machinery.

I notice that test_atomic::atomic_test parses the SPIR-V, validates it, generates WGSL, and then re-parses and re-validates the WGSL. This should never be necessary. If it is ever the case that passing a validated module to our WGSL back end generates code that our WGSL front end can't handle, that's a separate bug: a valid naga::Module should always generate valid output.

With that, I don't think there's anything that atomic_test does that the convert_spv_all snapshot test won't handle nicely.