Trigger to handle multiple unrelated updates safely

LeadTrigger

trigger LeadTrigger on Lead (After Update) {
    Map<ID, Lead> leadsToUpdate = new Map<ID, Lead>();//Create a Map to hold eventual updates
    
    //This is a very common pattern for iterating over the list of records
    //We use the size() method for Lists to handle each Lead in Trigger.new 
    
    for(Integer i = 0 ; i < trigger.new.size() ; i++){
        Lead old = trigger.old[i];
        Lead nw = trigger.new[i];//We cannot use "new" as our variable name, it's a reserved word.
        
        if(old.Status != nw.Status && nw.Status == 'Closed - Not Converted'){
            Lead l = new Lead(Id = nw.Id, Rating = 'Cold');
            leadsToUpdate.put(nw.Id, l);
        }
        
        if(old.Company != nw.Company && nw.Company == 'Our Competitor, Inc.'){
            if(leadsToUpdate.containsKey(nw.id)){
                leadsToUpdate.get(nw.Id).DoNotCall = True;
            } else {
                Lead l = new Lead(Id = nw.Id, DoNotCall = True);
                leadsToUpdate.put(nw.Id, l);                        
            }
        }
    }
    
    //BEST PRACTICE: Use size() again to check that you have records to be updated first
    if(leadsToUpdate.size() > 0){
        update leadsToUpdate.values();
    }
    
}

LeadTriggerTest

@isTest
public class LeadTriggerTest {
    
    public static testMethod void testOneClosedNotConverted(){
        Lead l = new Lead();
        l.LastName = 'Lead Last Name';
        l.Company = 'Lead Company';
        insert l;
        
		l.Status = 'Closed - Not Converted';
        update l;
        
        //Retrieve the new Status value for the updated Lead
        List<Lead> testLeads = [Select Rating from Lead];
        //and Assert that it has the value we expect
        System.assertEquals(testLeads[0].Rating, 'Cold');
        
        //Another way to do the same thing when there's only 1 record you want to test
        Lead testLead = [Select Rating from Lead Limit 1];
        System.assertEquals(testLead.Rating, 'Cold');
    }
    
    public static testMethod void testManyClosedNotConverted(){
        List<Lead> leadsToInsert = new List<Lead>();
        
        for(Integer i = 0 ; i < 2 ; i++){
            Lead l = new Lead();
            l.LastName = 'Lead Last Name' + i;
            l.Company = 'Lead Company' + i;
            leadsToInsert.add(l);
        }
        insert leadsToInsert;
        
        List<Lead> leadsToUpdate = new List<Lead>();
        for(Lead l : leadsToInsert){
            l.Status = 'Closed - Not Converted';
            leadsToUpdate.add(l);
        }
        update leadsToUpdate;
        
        List<Lead> testLeads = [Select Rating from Lead];
        for(Lead l : testLeads){
        	System.assertEquals(l.Rating, 'Cold');
        }
    }

    public static testMethod void testManyCompetitors(){

    }
    
    public static testMethod void testOtherCompanies(){
        
    }
    
    public static testMethod void testOtherStatuses(){
        
    }
}