Nextest support and update template

[Firestar99]

• tests get separate cache dirs so they don't clobber themselves if run in parallel
• cargo nextest run now works
• partial cherry-pick from Divide project into library and binary parts, round 2 #117
• update rust-gpu in shader template (in that old version spirv-tools fails to build, see rust-gpu master / spirv-tools fails to build on latest nix rust-gpu#370)

[Firestar99]

The template's spirv-std update included Rust-GPU/rust-gpu#380, which forces glam to 0.30.8+. But specifying that glam version breaks all older rustc_codegen_spirv versions, see 0.8.0 and 0.9.0 failing. So I switched to using the reexported glam from spirv-std.

[tombh]

Our main::cache_dir() function already has a clause for tests:

    Ok(if cfg!(test) {
        let thread_id = std::thread::current().id();
        let id = format!("{thread_id:?}").replace('(', "-").replace(')', "");
        dir.join("tests").join(id)
    } else {
        dir
    })

So either we need to delete that or, what about if we pass the function the tempdirpath, like cache_dir(Option<PathBuf>)? I think it'd be nice to still be getting test coverage for that function.

But now I'm wondering about tempdir's default use of Drop to automate the directory removal. What if shader_crate_test_path() returned tempfile::TempDir? Would that mean we don't need the thread-local TEMPDIR nor even the tests_teardown?