🏗️ [Core Feature]: Tests coverage and fuzzing

[brozorec]

What is the key feature we're aiming to implement?

We have adopted so far only unit and integration testing while the Soroban SDK offers out of the box a variety of testing opportunities such as fuzzing, differential or mutation testing among others. We have to take advantage of that, extend our testing strategies for the existing code and adhere to those practices for the future. Here's a list of tasks that will increase significantly tests quality:

• Authorizations testing: in progress with Add explicit auth tests for fungible token functions #84