Trying to deploy this trigger and as we all know there is a prerequisite for Salesforce for Code Coverage on your Trigger. I have created a Test Class for this Trigger and the problem I have is there are no Test Methods, I thought I established a method in the Test Class. 

Apex Test Class
 

@isTest
private class UpdateOpportunityProductMonthsToFulfill{

    @testSetup 
    static void testOLIUpdate()
    {
        // Obtain Apex governor limits and resources for this test
        Test.startTest();

        OpportunityLineItem oli = new OpportunityLineItem(Months_to_Fulfill__c='');
        insert oli;

        QuoteLineItem qli = new QuoteLineItem(OpportunityLineItemId=oli.Id);
        insert qli;

        qli.Year_Served_First__c='2018';
        qli.Months_Served_Y1__c='May;June';
        update qli;
        
       system.assertEquals(oli.Months_to_Fulfill__c, 'May 2018; June 2018');

        // Release governor limits and resources
        Test.stopTest();
    }
}

Apex Trigger​

trigger UpdateOpportunityProductMonthsToFulfill on QuoteLineItem (after insert, after update) {
    String monthsToFulfill = '';
    map<String, String> updateMap = new map<String, String>();
    
    // This is where the text for the Months_To_Fulfill__c field is built
    for(QuoteLineItem qli : Trigger.new){
        // To make a spelled-out list of months & years instead of the older "start through end"
        
        List<String> yearOneMonths = qli.Months_Served_Y1__c.replaceAll('None(;)?', '').split(';');
        for(String month : yearOneMonths){
            monthsToFulfill += ( monthsToFulfill.length()==0 ? '' : '; ' ) + month + ' ' + qli.Year_Served_First__c;
        }
        if(!(qli.Year_Served_Second__c == null || qli.Year_Served_Second__c.equals('None'))){
            List<String> yearTwoMonths = qli.Months_Served_Y2__c.replaceAll('None(;)?', '').split(';');
            for(String month : yearTwoMonths){
                monthsToFulfill += '; ' + month + ' ' + qli.Year_Served_Second__c;
            }
        }
        
        updateMap.put(qli.OpportunityLineItemId, monthsToFulfill);
    }
  
    for(String oliID : updateMap.keySet()){
        // This is the calculated value we saved from looping through the Trigger.new Collection
        monthsToFulfill = updateMap.get(oliID);
        
        // Get the OpportunityLineItems associated with this QuoteId
        List<OpportunityLineItem> oliList = [Select Id, OpportunityId, Months_To_Fulfill__c 
                                             From OpportunityLineItem Where Id  = :oliID ];
        
        List<OpportunityLineItem> olisToUpdate = new List<OpportunityLineItem>();
        
        if(oliList.size() > 0){
            for (OpportunityLineItem oli : oliList){
                // Populate this olisToUpdate list because we may 
                // want to get smarter about what's actually updated 
                // in the future.  For now, everything is updated.
                oli.Months_To_Fulfill__c = monthsToFulfill;
                olisToUpdate.add(oli);
            }
            
            // Batch update the OpportunityLineItems we identified earlier
            if(olisToUpdate.size() > 0){
                update olisToUpdate;
            }
        }
    }     
}