It seems that relaying on data in Roll-Up summary in apex code is not a good idea, as documentation states, this kind of field are calculated up to 30 minutes after child record save.
If you have test failing because of null or 0 instead of expected value and you are using Roll-Up summary then it means that field value is not yet calculated.

Hare is easy workaround, just check if it is Test run and query data directly:

Decimal calculatedRequestValue = 0.0; // It is currency
if (Test.isRunningTest()) {
    AggregateResult ar = [SELECT SUM(Line_item_value__c) totalReqValue FROM Order_Line_Item__c WHERE Order__c = :orderObject.Id];
    calculatedRequestValue = (Decimal) ar.get('totalReqValue');
} else {
    calculatedRequestValue = orderObject.Total_request_value__c;

I would recommend to avoid any usage of Roll-Up summary values for calculations especially in triggers, instead query data directly using SOQL.

© Paweł Woźniak