LeadProcessor.cls

public class LeadProcessor implements Database.Batchable<sObject>, Database.Stateful {

    /**
     * コンストラクタ
     */
    public LeadProcessor() {
    
    }

    /**
     * Start
     */
    public Database.QueryLocator start(Database.BatchableContext BC) {
        String query = 'SELECT Id FROM Lead';
        return Database.getQueryLocator (query);
    }
    
    /**
     * Execute
     */
    public void execute(Database.BatchableContext BC, List<Lead> leads) {
        for (Lead l : leads) {
            l.LeadSource  = 'Dreamforce';
        }
        update leads;
    }
    
    /**
     * Finish
     */
    public void finish(Database.BatchableContext BC) {
        
    }
}

LeadProcessorTest.cls

@isTest
private class LeadProcessorTest {

    private static User testAdminUser = new User(Id = UserInfo.getUserId());

    static testMethod void LeadProcessorTest() {
        
        System.runAs(testAdminUser) {
            
            List<Lead> leads = new List<Lead>();
            for (Integer i = 0; i < 200; i++) {
                leads.add(new Lead(LastName = 'Yoshikawa', Company = 'T.Yoshikawa Labs'));
            }
            insert leads;
            System.assertEquals(leads.size(), 200);

            Test.startTest();
            
            LeadProcessor batchable = new LeadProcessor();
            Database.executeBatch(batchable);
            
            Test.stopTest();
            
            List<Lead> results = [SELECT Id,LeadSource FROM Lead];
            for (Lead l : results) {
                System.assertEquals(l.LeadSource, 'Dreamforce');
            }
            System.assertEquals(results.size(), 200);
        }
    }
}