Add differential testing harness

[LegNeato]

The test harness compiles and runs Rust
shaders/kernels and compares the output with those produced by equivalent WGSL
shaders/kernels. Unlike traditional reference, golden, or snapshot tests—which check
against static outputs—differential testing compares multiple independent implementations of the same logic.
This approach is more robust because discrepancies between them are more likely to
indicate real bugs rather than issues with outdated reference files.

You can run difftests via cargo difftest. This is an alias set up in .cargo/config
for cargo run --release -p difftest --. You can filter to run specific tests by
passing the (partial) filenames to cargo difftest some_file_name.

[LegNeato]

Perhaps better DX is to have directories and just cargo run --release in each?

[LegNeato]

@Firestar99 FYI, I am going to revamp this a bit as it is a bit of a mess.

[LegNeato]

FWIW, I am almost ready to update this. It is much cleaner now :-)

[LegNeato]

I think this is ready for review. Sorry it is so big! I can break it down if need be.

The main changes:

1. Move compiletests under tests/ and add difftests there as well.
2. Add a difftest harness
3. Add a difftest sdk/library used by the binaries under test.
4. Add a simple compute difftest test using wgpu (one variant written in rust, one in wgsl).
5. Add to CI (on linux install software GPU so the tests will run)
6. Some docs in a README.

[LegNeato]

This is what a run with passing and failing tests looks like:

On failing tests, the outputs are grouped as you may have 4 variants where 3 are the same and 1 is different. In that case the 3 same will be grouped in the output so it is easier to understand what is going on.

[LegNeato]

Another option we could do rather than binaries, is sort of what I did before (have built in harnesses which are configured) but mirror wgpu's harness and config format. Perhaps that would allow us to leverage their test corpus by just writing the rust-gpu versions of their test inputs.

If I get my naga rust-gpu backend working, we could generate the rust-gpu code on the fly from the source wgpu test 🤔