test(core): Improve unit test performance for offline transport tests (#18628)

The `offline.test.ts` unit tests in core have been bugging me for a long
time because they take almost 30s to run locally. Which makes running
tests in watch mode in core quite annoying.

The core problem in these tests was that we used real timers and waited
for up to 25s in one test. This PR:
- uses vitest's fake timers instead of real timers
- removes the `waitUntil` calls in favor of advancing timers. Mostly by
the upper bound we specified before, though in some situations, I had to
use a lower value because the previous waitUntil condition was satisfied
earlier.
- re-enables the rate limiting test which was flaky before. I'm curious
if it'll flake again but my guess is that with the more strict fake
timers, it might just behave as we hope.

Now, the tests only take a couple of ms to complete.  

Closes #18630 (added automatically)

Author: Lms24