public with sharing class expensesLineItem{
    public final Expense_Line_Item__c el;
    
    public expensesLineItem(ApexPages.StandardController stdController){
        this.el = (Expense_Line_Item__c)stdController.getRecord();
    }
    public PageReference Dismiss(){
        el.Display_Alert__c=false;
        update el;
        PageReference page = ApexPages.currentPage();
        page.setRedirect(true);
        return page;
    }
}


Test class:

@isTest
public class expensesLineItem_Test{
  @testSetup
  static void setupTestData()
  {
    Expense__c exp = new Expense__c();
    exp.Period_From__c = system.today();
    exp.Period_To__c =system.today();
    insert exp;
    System.assertNotEquals(null, exp.Id); 
    
    Expense_Line_Item__c expLine = new Expense_Line_Item__c();
    expLine.Expense__c = exp.Id;
    expLine.Expense_Head__c='Food' ;
    expLine.Cost_Head__c= 'Campaign' ;
    expLine.Amount__c= 10000;
    System.assertNotEquals(null, expLine.Id);
    insert expLine;
 }
  
  @isTest 
  static void testCall()
  {
    Expense_Line_Item__c expLine  =  [SELECT Id,Name,Amount__c,Bill_Available__c,Cost_Head__c,Expense_Head__c,Expense__c from Expense_Line_Item__c][0];
   // System.assertEquals(true,expense_line_item_Obj.size()>0);
   // List<Expense__c> expense_Obj  =  [SELECT Id from Expense__c];
    //System.assertEquals(true,expense_Obj.size()>0);
    ApexPages.StandardController stdCon = new ApexPages.StandardController(expLine);
    expensesLineItem obj01 = new  expensesLineItem(stdCon);
    
    //expensesLineItem obj01 = new expensesLineItem(new ApexPages.StandardController(expense_line_item_Obj[0]));
    obj01.Dismiss();
  }
}