…n#20503.

We're using two different functions to atomically decrement a reference count,
the native `atomic_fetch_sub` and the managed `Interlocked.Decrement`.

Unfortunately the return value is not the same: `atomic_fetch_sub` returns the
original value before the subtraction, while `Interlocked.Decrement` returns
the subtracted value, while our code assumed the functions behaved the same.
This resulted in a memory leak, because we'd incorrectly expect `0` to be
returned from `atomic_fetch_sub` when the reference count reaches zero, and
thus not detect when the descriptor a block should be freed.

The fix is to update the expected return value from `atomic_fetch_sub` to be
`1` instead of `0`.

Fixes xamarin#20503.

…se/8.0.1xx-xcode15.1-2024-05-06

…se/8.0.1xx-xcode15.1-2024-05-06