A: Trigger

//Ray : On every Insert Opportunity, create a Opportunity Line Item Automatically
trigger CreateOLIonInsertOpp on Opportunity(After insert) {
  List < Opportunity_Line_Item__c > list_opportunity_line_item = new list < Opportunity_Line_Item__c > ();
  //for every new Opportunity inserted
  if (Trigger.IsInsert) {
    for (Opportunity vopportunity: [SELECT Id,Product_Name__c,Product_Pitched__c,Trial_Created__c,Trial_Created_date__c
        FROM Opportunity WHERE Id IN: trigger.New]) {
      //create a Product Map
      Map < String, Id > productMap = new map < String, Id > ();
      for (Product_Master__c vproduct: [select id, Name From Product_Master__c]) {
        productMap.put(vproduct.Name, vproduct.Id);
      }
      //check for a valid Product_Pitched__c 
      if (vopportunity.Product_Pitched__c != null) {
       system.debug('------------>>>>>>>For every product in OLI creation' + vopportunity.Product_Pitched__c);
       for (String singleProduct: vopportunity.Product_Pitched__c.split(';')) {
          system.debug('------------>>>>>> product' + singleProduct);
          if (productMap.containskey(singleProduct)) {
            system.debug('------------>>>>>>exists,');
            //create a line item in case of a Product_Pitched__c is found in Product_Master__c
            Opportunity_Line_Item__c vopportunity_line_item = new Opportunity_Line_Item__c();
            vopportunity_line_item.Opportunity__c = vopportunity.Id;
            vopportunity_line_item.Product_Name__c = productMap.get(singleProduct);
            if(vopportunity.Trial_Created__c == 'Yes') {
              vopportunity_line_item.Trial__c = true;
              vopportunity_line_item.Trial_Start_Date__c = vopportunity.Trial_Created_date__c !=null ? vopportunity.Trial_Created_date__c : DateTime.now();
            }
            system.debug('------------>>>>>> create a new opportunitylineitem' + vopportunity_line_item);
            list_opportunity_line_item.add(vopportunity_line_item);
          }
        }
      }
      // insert the newly created OLIs 
      if (list_opportunity_line_item.size() > 0) {
        insert list_opportunity_line_item;
      }
    }
  }
}

// Creates a product line item(custom child object on opportunity) with details of product. 

B. Test Class

@isTest
public class createOLIonOPPInsert_Test {
  public static testMethod void testolicreate() {
    Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator']; 
    user intuser = new User(Alias = 'standt', Email='standarduser@testorg.com', 
      EmailEncodingKey='UTF-8', LastName='Integration User', LanguageLocaleKey='en_US', 
      LocaleSidKey='en_US', ProfileId = p.Id, 
      TimeZoneSidKey='America/Los_Angeles', UserName='intuser@practo.com');
    insert intuser;

    system.runAs(intuser) {
        //List<Territory_Mapping__c> list_Territory_Name = [Select Id From Territory_Mapping__c Where Territory_Name__c = 'Rest of India' Limit 1];
        String strRecordTypeId = [Select Id From RecordType Where SobjectType = 'Opportunity' and Name = 'Ray ROI'].Id;


        Account newaccount = new Account(
          Name = 'test opp Jul 171435',
          Practice_Category__c =  'Clinic',
          Ray_Practice_ID__c = '171435',
          Is_Computer_Available__c = 'Don\'t Know',
          Is_Internet_Availabe__c = 'Don\'t Know',
          Is_Receiptionist_Available__c = 'Don\'t Know',
          Lead_Type__c = 'Unpaid',
          Primary_Contact__c = '8989787898',
          Country_Code__c = 'India Only'
        );
        insert newaccount;

        Opportunity newopp = new Opportunity(
          RecordTypeId = strRecordTypeId,
            //Prod '012280000011DFA', Test : 0120k0000004NvXAAU
          //opportunity_id, //RAY ROI
          Name =  'OPP-'+ System.today().format(),
          Accountid = newaccount.Id,
          Product_Name__c  = 'Ray',
          Product_Pitched__c = 'Ray CM Atom',
          Closedate = System.today() + 7 ,
          StageName = 'Trial'
        );
        insert newopp;
    }
  }
}

//Creates and acccount and opportunity with mandatory fields filled in and expectation is that it will activate trigger on insert.

Pls Help!! @ankit maini, others