Need to create Test Class for Trigger

Try Below test Class

@isTest
public class TestTaskTriggers {
    @isTest
    public static void unitTest(){
        Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator'];
        User usr = new User(LastName = 'sample',
                            FirstName='sam',
                            Alias = 'simplSam',
                            Email = 'ssamsimple@asdf.com',
                            Username = 'ssamsimple@asdf.com',
                            ProfileId = p.id,
                            TimeZoneSidKey = 'GMT',
                            LanguageLocaleKey = 'en_US',
                            EmailEncodingKey = 'UTF-8',
                            LocaleSidKey = 'en_US'
                           );
        insert usr;
        system.runAs(usr){
            Test.startTest();  
            Account acc = new Account();
            acc.name='Test Account';
            insert acc;
            Trial__c trial = new Trial__c (Name = 'Test', Matter__c='a172h000001FLYZAA4');
            insert trial;
            Task tsk = new Task();
            tsk.Subject = trial.Name + ' Test Task';
            tsk.Type = 'Email';
            tsk.Task_Reference__c = 'L-739-5';
            tsk.ActivityDate = System.today().addMonths(1);
            tsk.Description = 'Test Task';
            tsk.OwnerId = usr.Id;
            tsk.WhatId = acc.Id;
            insert tsk;
            task.Status='Completed'; 
            update task;	   
            Test.stopTest();  
        }
    }
}