The owner of contract DormantAsset is dormant_asset_factory and you are trying to execute setHeir() function from accounts[0]. Function setHeir() has modifier onlyOwner and we already know that the owner is factory contract. You should implement function callSetHeir() in your factory contract and then call it from your test.

Hope it helps.

Bonus: more readable way to write function with returned value

function newDormantAsset() public returns(address c) {
     DormantAsset c = new DormantAsset(_heartbeatTimeout);
     dormantAssets.push(c);
}

The owner of contract DormantAsset is dormant_asset_factory and you are trying to execute setHeir() function from accounts[0]. Function setHeir() has modifier onlyOwner and we already know that the owner is factory contract. You should implement function callSetHeir() in your factory contract and then call it from your test.

Hope it helps.

Bonus: more readable way to write function with returned value

function newDormantAsset() public returns(DormantAsset c) {
     c = new DormantAsset(_heartbeatTimeout);
     dormantAssets.push(c);
}