refactor: `vertex_index_common`: remove footguns in `tests` matrix init.

[ErichDonGubler]

Connections

Addresses non-blocking feedback raised in https://github.com/gfx-rs/wgpu/pull/5773 (1, 2).

Description

Remove a couple of footguns around allocation and exhaustive matrices for tests using the vertex_index_common helper.

Testing

I've used assert_eq!(…) checks between new and old values (not committed here) to ensure that we haven't regressed, so I feel fairly confident. 🙂 Attached: test.zip

Checklist

• [x] Run cargo fmt.
• [x] Run cargo clippy. If applicable, add:
  • [x] --target wasm32-unknown-unknown
  • [x] --target wasm32-unknown-emscripten
• [x] Run cargo xtask test to run tests.
• [x] Add change to CHANGELOG.md. See simple instructions inside file. Not necessary.

[jimblandy]

Sorry about the confusion - I didn't realize this PR was against vertexPullByDefault and not trunk.