Update sponsored transaction tutorial - test cases section

[DOBEN]

Task description Cis3-sponsored-transaction test cases have been significantly changed.

https://developer.concordium.software/en/mainnet/smart-contracts/tutorials/sponsoredTransactions/sponsoredTransactionsSmartContract.html#test[…]tion

Sub-tasks

• [ ] Update the section testing the signature verification with up-to-date function names and screenshots
• [ ] Add section explaining how the inside_signature generation works in the test cases.
• [ ] Explain when to use (inside/outside signatures) e.g. test_inside_signature_permit_transfer vs test_outside_signature_permit_transfer. Mention that test_inside_signature_permit_transfer should be the recommended approach.

[dg-concordium]

Please also see #907 as part of that also pertains to updating the sponsored transactions tutorial.