Add more reflection tests for Handle

[greeble-dev]

Objective

Build on #24048 to improve test coverage for Handle reflection, and also repro #24111.

Solution

Add various tests. Includes a test for valid cases of from_reflect and try_apply - this is relevant since from_reflect now has a custom implementation.

I've snuck in a few cleanups too:

• Avoid multiple tests repeating the same imports.
• Fix an accidentally duplicated comment.

Testing

cargo test -p bevy_asset

[alice-i-cecile]

Broken by the force push noted in #24130; you'll need to clean up the Git history per @cart's message.