Cover example contracts with tests

Is your feature request related to a problem? Please describe.

Some of our example contracts are not tested although they are complicated. In fact, only NNS contract has some unit-tests.

Describe the solution you'd like

At least complicated contracts like nft-nd and nft-d must be covered with neotest unit tests. Ideally, all of them must be covered with tests. First of all, these contracts are examples for other SC developers. Secondly, when we extend these contracts, we need to have a way to check that extension works properly, e.g. for situations like in https://github.com/nspcc-dev/neo-go/pull/3560.

Describe alternatives you've considered

No alternatives, tests are must-have.