test(Keys_Nullifier): Showcase full nullifier key rotation

[LHerskind]

The key registry allows rotating the nullifier key, however, the caching done in the private_context in request_app_nullifier_secret_key means that only one key could be requested within the same call. This means that you cannot spend notes with different nullifier keys during the same call, as the assertions will fail.

I propose that we cache the latest, such that we don't insert repetitive requests, but allow making multiple request to ensure that we properly support key rotation.

To showcase the issue, I think it is useful to create a separate test that is receiving tokens, rotating the key and then receiving more and finally trying to send it all.

When showcased we can fix it as well.

Addressing this fits well with #6313.

[LHerskind]

Should wait until #6130 is in to properly test with tokens.