Add test to verify StoredValue persistence across instances

[Salander7]

I added a new test that verifies @Storedvalue properly persists data across multiple instances.

The test verifies that:

1. When a value is stored using one instance, it can be retrieved using another instance with the same key
2. Changes made in the new instance are reflected in the original instance
3. The persistence works correctly across different instances of StoredValue

This helps ensure the persistence behaviour of @Storedvalue works as expected when the same key is used in different places.

[mergesort]

Hey @Salander7, thank you so much for the contribution! I'm currently on my honeymoon so realistically I won't be able to look at this too in depth until I'm back closer to the end of the month, but at a glance this looks like a good idea and the right idea. Before merging this in could I ask you to add a similar test for @SecurelyStoredValue? (You can skip @AsyncStoredValue since it's being deprecated in Boutique 3.0.)

Thank you so much!

[Salander7]

Thank you for the feedback! I've added a test for @SecurelyStoredValue following the same pattern. Enjoy your honeymoon! 🎉