Some_Test.cls

@isTest
public with sharing class Some_Test {
    private Account acc {get; set;}

    @testSetup static void setup() {
        acc = new Account(Name = 'account1');
        insert acc;
        System.debug(acc.Id); // We can confirm Account Id on the debug log.
    }

    @isTest static void testMethod1() {
        System.debug(acc.Id); // We'll get an error. NullPointerException occur!!
    }
}