[Building Sync-ups Tutorial] fixes in "Testing the sync-up detail" chapter

[FredericRuaudel]

Hi!

Following my last PR, I'm currently reviewing the "Building Sync-ups" tutorial and so far, there is a lot of changes so I've splitted up into one PR per chapter to ease the review of them, continuing with the "Testing the sync-up detail" chapter here…

Changes Made:

• Clarified Tutorial Introduction: The introduction of the SyncUpDetail tutorial has been updated to take into account the change from delegate pattern to Shared pattern.
• Refactored Test Code: The tutorial's test code has been modified to utilize the modify function for nested state updates, fixing errors explained in https://github.com/pointfreeco/swift-composable-architecture/discussions/3158. Additionally, the withLock function has been implemented to fix the associated compilation errors.
• Restored Delete Flow Testing: The tutorial has been updated to include testing for the delete flow using the @Shared property, that was left for a future update I guess. Here again, I've tried my best to fit your tones and vocabulary in the explanation but feel free to update the copyrighting as you see fit 🙂