Missing tests

This is a consolidation of prior issues about missing tests:

- [ ] If no valid, up to date, price feed then No new shorts No forced settling No margin calls #27
- [ ] Implement unit test for checkpoints #40
- [x] Implement unit test for MIA backed by asset other than CORE #41
- [ ] Test renewal of annual membership #42
- [ ] Implement unit tests for all vesting policies #43
- [ ] Implement unit test for get_impacted_accounts() #44
- [ ] Implement unit test for all assert predicates #45
- [ ] Implement unit test for withdrawing balance belonging to PTS address #46
- [ ] Implement unit test for whitelist_markets and blacklist_markets #47
- [x] Implement unit test to change backing asset of MIA #48
- [ ] Implement unit test for withdraw permission on whitelisted asset #49
- [x] Implement unit test for proposed tx already containing some owner / active approvals #50
- [ ] Implement unit test for witness budget running out #51
- [ ] Test worker pay computation when maintenance interval is not 1 day #52
- [x] Implement unit test for "fill call" case in check_call_orders #53
- [ ] Implement unit test for custom_operation #54
- [ ] Implement unit test for account_transfer_operation #55
- [ ] Investigate cdd_vesting_policy::on_deposit_vested coverage #56
- [x] Test core_exchange_rate update logic in update_expired_feeds() #57
- [ ] Implement unit test to verify time of next block #58
- [ ] witness_update operation needs tested #70
- [x] Implement unit tests for advanced proposal usage #83

Start by selecting one unchecked target, such as checkpoints, vesting policies, get_impacted_accounts(), or witness_update, then locate its existing implementation and related tests in the C++ repository. Run the relevant test suite before changing anything. Done means the selected behavior has a focused unit test and passes without regressing existing tests.

Written by the indexing model from the issue text.