fix mockRejectedValue throwing error if not fulfilled on next tick

[gpfunk]

What does this PR do?

This lets mockRejectedValue() and mockRejectedValueOnce() not throw errors if they're not fulfilled / resolved immediately after being set. This would match jest's implementation

To do this I switched up the logic so rather than returning a rejected promise, it returns a function that returns a rejected promise

Fixes #10437

I'm not that familiar with c++ or this codebase, so if theres a cleaner way to implement this please let me know! Thanks

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

I wrote automated tests to ensure mocks that weren't called and fullfilled would not throw errors