Add test that repros #11111 (different loader settings produce same asset)

[greeble-dev]

Objective

Add a test that reproduces #11111 (and partially #18267). The bug is that asset loader settings are effectively ignored if the same asset is loaded multiple times with different settings.

Solution

Add a unit test to bevy_assets/lib.rs. The test will be marked as #[ignore] until #11111 is fixed.

// Load the same asset with different settings.

let handle_1 = load(asset_server, "test.u8", 1);
let handle_2 = load(asset_server, "test.u8", 2);

// Handles should be different.

assert_ne!(handle_1, handle_2);

Concerns

I'm not 100% sure that the current behaviour is actually broken - I can't see anything in the asset system design docs that explicitly says different settings should create different asset ids.

UPDATE: Sentiment from issue comments and discord varies between "bug" and "undesirable consequence of design decisions, alternatives should be explored". So I've concluded that the test is valid and desirable.

Testing

cargo test -p bevy_asset --features multi_threaded

# Or to repro the issue:
cargo test -p bevy_asset --features multi_threaded -- --ignored

[alice-i-cecile]

I think this behavior is a bug: doing distinct things with the same asset is a relatively common pattern.