feat(iota-json-rpc-tests): add governance api tests

[samuel-rufi]

Description of change

Added the remaining GovernanceApi tests for the iota-json-rpc-tests crate.

It appeared that there is an issue in the unstaking and timelocked unstaking feature. The test_unstaking test and
test_timelocked_unstaking test provide coverage for the issue. Both tests have been ignored for now as further investigation is needed as it seems to be a more underlying issue to solve - which is outside of scope of this PR. Related tracking issue: #3007

Links to any relevant issues

Fixes #2917

Type of change

• Tests

How the change has been tested

cargo simtest -p iota-json-rpc-tests --test governance_api

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[sergiupopescu199]

lgtm ✨