Add support for waitUntil accepting an async function?

Hi,
I wanted to use `waitUntil` and pass an async function as the callback.
It looks like this [isn't currently supported](https://github.com/emberjs/ember-test-helpers/blob/27389be9e42b6f1c2c295f8c8d3ba6517f1a80f6/addon-test-support/%40ember/test-helpers/wait-until.ts#L51). Is this something you would be interested in supporting?

If so I can try to open a PR which implements Promise support in `waitUntil`.

Read addon-test-support/@ember/test-helpers/wait-until.ts at line 51 to understand the current callback handling and why async functions are unsupported. Define coverage for Promise-returning callbacks and consider the existing waitUntil behavior; done means async callbacks are accepted without regressing synchronous callback support.

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