trigger attachLeadInTaskToAccount on Task (before update) {
  // Get Lead Ids from all of the triggered Tasks
  set<id> leadIds = new set<id>();
  for(Task myTask :trigger.new) {
    leadIds.add(myTask.whoId);
  }

  // Get Leads and build a map of <LeadId,Lead_Accounts__c>
  map<id,id> mLeadAccountIds = new map<id,id>();
  list<Lead> leads = [SELECT id, Lead_Accounts__c FROM Lead WHERE Id IN :leadIds];
  for(Lead l :leads) {
    mLeadAccountIds.put(l.id,l.Lead_Accounts__c);
  }

  // Update triggered tasks with Account Id
  for(Task t :trigger.new) {
    if(mLeadAccountIds.containsKey(t.whoId)) {
      t.whatId = mLeadAccountIds.get(t.whoId);
          }
  }

}

@isTest
public class attachLeadInTaskToAccount_Test
{
      public static testmethod void testinsert()
      {
            
list<Lead> leads = [SELECT id, Lead_Accounts__c FROM Lead WHERE Lead_Accounts__c !=null AND Lead_Accounts__r.Name = 'test'];

if(leads.size() > 0) {

String lead1 = leads[1].id;
String lead2 = leads[2].id;
String lead3 = leads[3].id;

Task task= new task();
task.Subject='Message Sent 1';
task.status='Completed';
task.Whoid= lead1;
insert task;

Task task1= new task();
task1.Subject='Call';
task1.status='Completed';
task1.Whoid= lead2;
insert task1;

Task task2= new task();
task2.Subject='Message Sent 3';
task2.status='Completed';
task2.Whoid= lead3;
insert task2;

task.Subject = 'Closed';
update(task);

task1.Subject = 'Closed';
update(task1);

task2.Subject = 'Closed';
update(task2);

}
        }

}