(WIP) Add tests for empty buffers and map ranges

[nical]

Checklist

• [x] Run cargo clippy.
• [ ] Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• [ ] Add change to CHANGELOG.md. See simple instructions inside file.

Connections

There are a couple of fixes related to zero-sized buffers in https://github.com/gfx-rs/wgpu/pull/2877,

Description

This PR adds some tests to verify the fixes in https://github.com/gfx-rs/wgpu/pull/2877. The tests aren't quite ready to land because creating a zero-sized buffer slice currently panics, I'm trying to figure out why that is. The assertion was added in https://github.com/gfx-rs/wgpu/commit/2bc930d905c990aeb3c25cd9fc37bd04a256e805

Testing

This PR is all about testing.