Add test scenario for account to multiSig rekey transaction

[sczembor]

Closes #x

Proposed Changes

• added unit test to check if the spend address was changed to multiSig address
• added unit test to check if it is possible to spend from the account using one of the multiSig addresses

TODO

• [x] Add unit tests
• [ ] Add more test scenarios:
  • [ ] rekey multiSig to account
• [ ] Verify if the tests are correct
• [ ] Fix the potential problems

[robert-zaremba]

Should we change this to a draft PR until all subtasks are solved?