test: Fix flaky session tracker tests

[philipphofmann]

📜 Description

Testing with NSNotificationCenter in CI leads to flaky tests.
Therefore, we use a wrapper around NSNotificationCenter to not
depend on it. Instead, we call the methods NSNotificationCenter
would call with Dynamic and ensure that sut properly subscribes
to NSNotificationCenter.

#skip-changelog

💡 Motivation and Context

Fixes #2062

💚 How did you test it?

Unit tests.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• Review from the native team if needed
• No breaking changes

🔮 Next steps

[brustolin]

LGTM

[armcknight]

Looking back at my attempt to use expectations to wait for the async delivery of these notifications (edd4706), and I just realized I never actually fulfilled them in those test cases 😕 Gotta use expectations (and correctly haha) for async api like this

[philipphofmann]

I also tried the expectations, but it didn't work. I have no clue what the issue is, but not it works.