Use Mockchain to test multisig actions

### What should be done?

Currently, live testnet is used for the test(s) present in [multisig_engine_test.rs](https://github.com/0xMiden/MultiSig/blob/main/crates/coordinator/engine/tests/multisig_engine_test.rs). This results in test(s) being extremely flaky with success rate being not more than ten percent.

### How should it be done?

Follow the guide at [miden docs](https://docs.miden.xyz/quick-start/your-first-smart-contract/test/#1-setting-up-the-mockchain-builder).

### When is this task done?

The test(s) don't need to interact with the live testnet.

### Additional context

_No response_