Support assertSnapshotSync

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

I'm working on RxJS, which also use the TestScheduler provide from RxJS.
But there is a limitation when I use TestScheduler doing the test, which cannot have Promise in the test.
https://rxjs.dev/guide/testing/marble-testing#rxjs-code-that-consumes-promises-cannot-be-directly-tested

**Describe the solution you'd like**

I try to implement by myself, to use Deno.mkdirSync(), Deno.writeTextFileSync(), Deno.readTextFileSync(), and it works in simple case.
But I realize it's hard becuase there are so many situations need to handle, it's not just simply read/write file and compare the content.

If it can support from standard library will be very good.