Add a test to check that we're only launching one load task when loading subassets.

[andriyDev]

Objective

• Fixes many_foxes and other animation examples have stopped working #20383.
• Fixes Loading an asset and one of its subassets causes two loads of the same asset to occur. #12756.

Solution

• Count the number of loads for a root asset that are currently happening. Note this needs to count assets by path and type ID, since you could be loading an asset path multiple times with different types.
• Don't launch a new load for a root asset if its number of loads is positive.
• Add some special handling for load_untyped_async to allow it to load even if the number of loads is already positive (since we need the handle, which means awaiting the async task which needs to perform the actual load).

Testing

• Added a test to check that we only launch one task.
  • This test is brittle. We don't have any way to count how many tasks are currently running, so the best we can do is just hope the tasks finish quickly and stall until they are done.
• Ran the many_foxes example and it now works correctly!