Show how to tests shaders using Bevy

## What problem does this solve or what need does it fill?

Shaders are hard to tests, but with a little work unit tests are possible.

## What solution would you like?

Take inspiration from wgpu shader tests https://github.com/gfx-rs/wgpu/tree/trunk/tests/tests/wgpu-gpu/shader

A test takes a compute shader template, plugs the types / code, calls the shader, read back the result, and assert the result. See https://github.com/gfx-rs/wgpu/blob/trunk/tests/tests/wgpu-gpu/shader/mod.rs#L200