Rewrite tests of all example contracts to use the testing library

[Bargsteen]

Task description

Rewrite tests of all example contracts to use the new(ish) testing library. The examples with quick check tests won't be migrated yet, as the testing lib does not yet support quick check tests.

Depends on:

• https://github.com/Concordium/concordium-rust-smart-contracts/pull/336

Subtasks:

• [x] account-signature-checks (none)
• [x] auction
• [x] cis2-multi
• [x] cis2-multi-royalties
• [x] cis2-nft
• [x] cis2-wccd
• [x] cis3-nft-sponsored-txs
• [x] counter-notify
• [ ] credential-registry (won't do, uses quickcheck)
• [x] eSealing
• [x] fib
• [x] icecream
• [x] memo (none)
• [x] nametoken
• [ ] offchain-transfers (won't do, needs larger contract changes)
  • In particular, we need to add some more view entrypoints to be able to test it completely. In the interest of time, we'll skip that for now.
• [x] piggybank
• [x] proxy
• [x] recorder
• [x] signature-verifier
• [x] smart-contract-upgrade
• [x] transfer-policy-check
• [ ] two-step-transfer (won't do, uses quickcheck)
• [x] voting