Trigger:
trigger Rfleet_DeleteRecords_Draft on Opportunity_car_set__c(before delete) {
    set < id > BId = new set < id > ();
    set < id > OpId= new set <id>();
    if (Trigger.isDelete && Trigger.isBefore) {
        Id devRecordTypeId = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('RFLEET-OPP-Algeria-RT').getRecordTypeId();

        for (Opportunity_car_set__c rec: trigger.old) {
            BId.add(rec.Opportunity__c);
            OpId.add(rec.id);
            List < Opportunity > ListOpp = [select StageName, RecordTypeId from Opportunity where id = : BId];
           for (Opportunity opp: ListOpp) {
                if (opp.RecordTypeId == devRecordTypeId) {
                    if (opp.StageName == 'Qualification' || opp.StageName == 'Prospecting') {
                        rec.adderror('You cannot delete this record!');
                    }
                }
            }
        }
      
    }
     delete [SELECT id FROM Service__c where Opportunity_car_set__c =:OpId]; 
}

Test class:
@isTest
private class  Rfleet_DeleteRecords_Draft_Test{
  static testMethod void Rfleet_DeleteRecords_Draft(){
       test.startTest();    
    
        Opportunity opp= new Opportunity();
        opp.name='Prabu';
        opp.stagename='Qualification';
        opp.CloseDate=system.Today();
        insert opp;
        opp.name='kiran';
        //opp.StageName = 'Prospecting';
        update opp;
      system.assertEquals('kiran',opp.name);
      
    
        Opportunity_car_set__c oppCarSet = new Opportunity_car_set__c();
        oppCarSet.name='Opp';
        oppCarSet.Opportunity__c =opp.id;
        oppCarSet.Quantity__c =2;
        insert oppCarSet;
        oppCarSet.Quantity__c =3;
      update oppCarSet;
      system.assertEquals(3,oppCarSet.Quantity__c);
     
        
        Service__c ser = new Service__c();
        ser.Opportunity_car_set__c = oppCarSet.id;
        ser.Quantity__c = 1;
        ser.Price_HT__c = 10;
        insert ser;
      ser.Price_HT__c = 20;
      delete ser;
      
      system.assertEquals(20,ser.Price_HT__c);
      test.stopTest();
      
        
    }
}