RemovedComponent removal detection has no tests

[alice-i-cecile]

While trying to poke into how this works, and investigate a possible bug, I found that RemovedComponent just has no tests, at all.

The example is nice, but not automated.

We need to verify that removal detection reports the correct entities, and the correct number of entities on both component removal and removal of entities with that particular component.

[hymm]

There is one test here

bevy/crates/bevy_ecs/src/system/mod.rs

Lines 453 to 479 in 6a8a8c9

	fn remove_tracking() {
	let mut world = World::new();
	struct Despawned(Entity);
	let a = world.spawn().insert_bundle((W("abc"), W(123))).id();
	world.spawn().insert_bundle((W("abc"), W(123)));
	world.insert_resource(false);
	world.insert_resource(Despawned(a));
	world.entity_mut(a).despawn();
	fn validate_removed(
	removed_i32: RemovedComponents<W<i32>>,
	despawned: Res<Despawned>,
	mut ran: ResMut<bool>,
	) {
	assert_eq!(
	removed_i32.iter().collect::<Vec<_>>(),
	&[despawned.0],
	"despawning results in 'removed component' state"
	);
	*ran = true;
	}
	run_system(&mut world, validate_removed);
	assert!(*world.get_resource::<bool>().unwrap(), "system ran");
	}

It tests with despawning an entity. Probably need to add one for removing a component

[alice-i-cecile]

Thanks; I'll add another test quick, improve the docs and make a PR.