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
• 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
• Implement unit test to change backing asset of MIA #48
• Implement unit test for withdraw permission on whitelisted asset #49
• 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
• 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
• 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
• Implement unit tests for advanced proposal usage #83