`assertEquals` could return `Promise` for `Blob` comparison

**Is your feature request related to a problem? Please describe.**

Documentation of `assertEquals` says:

> Note: When comparing `Blob` objects, you should first convert them to `Uint8Array` using the `Blob.bytes()` method and then compare their contents.

But what if one wants to compare whole objects that must contain `Blob`s as property values in a quite deep level of nesting?

**Describe the solution you'd like**

`assertEquals` should be able to return `Promise` to defer such comparison.

In , @kt3k said that changing `assertEquals` to async would be a large impact, but it seems unclear how large; the current return type of `assertEquals` is just `void`, so no one is supposed to use the return value. And of course non-async part of assertion can still be done synchronously before returning a `Promise`. Users who don't use `Blob`s wouldn't have to `await assertEquals`.

**Describe alternatives you've considered**

To write `assertEquals` for every single leaf property of the objects in concern, which is catastrophically painful.