Update .wast tests for async keyword

[fitzgen]

No description provided.

[lukewagner]

Does the recent merge of #646 (this commit) subsume this?

[fitzgen]

I don't think it fully subsumes this one, as I still get these async keyword errors when updating Wasmtime's git submodule to the latest main:

---- CraneliftNative//home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast ----
failed to run spec test with default engine

Caused by:
    0: failed directive on /home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast:9
    1: failed to parse WebAssembly module
    2: the `async` canonical option requires an async function type (at offset 0xa29)


---- CraneliftPulley//home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast ----
failed to run spec test with default engine

Caused by:
    0: failed directive on /home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast:9
    1: failed to parse WebAssembly module
    2: the `async` canonical option requires an async function type (at offset 0xa29)


---- CraneliftNative/pooling//home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast ----
failed to run spec test with default engine

Caused by:
    0: failed directive on /home/nick/wasmtime/tests/component-model/test/async/drop-cross-task-borrow.wast:9
    1: failed to parse WebAssembly module
    2: the `async` canonical option requires an async function type (at offset 0xa29)

[fitzgen]

Will make a new patch on top of main.