Refactor unit tests handling timers

[quentinderoubaix]

Context
Some of our unit tests are creating and waiting for timers (cssTransition, promise) but are not using fake timers.
So not only do those tests take some time, but they were also flaky.
It became even worse after upgrade to vitest 1.3.1.

Implementation
Investigate the reason of this unreliability and remove the FIXMEs created in this commit
Potentially use vitest fake timers.