Make GatedReader a test-only symbol, and allow all bevy_asset tests to all run single threaded.

[andriyDev]

Objective

• Some tests have been disabled in single-threaded mode.
• This was because the channel blocks the executing thread when in the asset reader. But that means we never get to the test line to open the gate! So we need multithreading just to let the reader block and the test can make progress in the background.

Solution

• The solution to allowing all tests to run single threaded is to make GatedReader await for gates to open rather than blocking on it.
• This however requires adding async-channel (already in tree). To avoid this additional dependency in most cases, I made this a dev-dependency.
• However, this now means that GatedReader can only be used in tests (in bevy_asset). We could make this a feature flag, but it seems highly unlikely that real users need a GatedReader - and if they need it for their test, they can just fork the GatedReader.

Testing

• The bevy_asset tests pass without setting the --features multi_threaded flag!

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Migration Guide

bevy_asset::io::gated::GatedReader and bevy_asset::io::gated::GatedOpener are no longer accessible to users.

[bushrat011899]

Nice! async_channel is fully no_std anyway, so it's inclusion as a true dependency in the future wouldn't be an issue.