Improve multi-tests

[maurolacy]

Follow-up on the current state of testing.

TODO:

• [x] #146
• [x] Remove Mocks / stub contracts. Replace mocks by the real contracts, and adjust tests.
  • [x] vault: cross_staking_mock
  • [x] vault: local_staking_mock
  • [ ] converter: virtual_staking_mock
  • [x] native_staking: local_staking_proxy
• [x] Add missing test cases / improve the state of unit / integration testing. Consumer side contracts in particular lack a good of number of tests.
  • #131
  • #132
• [ ] Send IBC messages when possible. Perhaps by modifying cw-multi-test so that it doesn't fail on unknown IBC messages, but just log them and ignore them.
• [ ] Improve test (through cfg_attr(test, msg(exec))) code. See https://github.com/CosmWasm/sylvia/issues/206.
• [ ] Improve sudo handling / tests, with new sylvia support for it.

[maurolacy]

Closing as done. Will create follow-up issues for the pending bullet points.

[maurolacy]

Re-opening, as virtual-staking-mock replacement and some other tasks are pending.