chore(test): use fakeTimers with setTimeout in buffer, skipUntil and window

[jsaguet]

Description:

After checking out the rxjs repo and running the unit tests, I noticed these 3 tests were always failing. (I'm running Windows 10 with Node 20)

The issue is probably that setTimeout does not necessarily execute at the specified delay.
I tried to reproduce the test in StackBlitz and I had the exact same issue when using timeouts so close to each other.

The PR updates these tests to use fake timers to make sure the tests work as expected.

[pmoleri]

I had the same issue when running the tests on Windows 10 node@18.18.2.
It's very annoying to contribute when there are unrelated failing tests.

I came across this interesting stackoverflow question: https://stackoverflow.com/questions/65177373/odd-behavior-with-setinterval-in-node-js-windows-only-works-in-linux that seems to explain the issue.

fakeTimers seems to be the way to go if we don't want to slow down the tests to be consistent in different platforms.