tests: Added tests for SerializableFelt252Dict

[akhercha]

Pull Request type

Please add the labels corresponding to the type of changes your PR introduces:

• Testing

What is the current behavior?

Resolves: #NA

What is the new behavior?

• Added tests for SerializableDict to check if we can easily compare Items (single & spans).

Does this introduce a breaking change?

No

Other informations

@Sk8erboi84