How to inject PersistenceContext during unit testing?

Without a container like Spring (or something like Unitils - which is Spring based), you will have to inject the entity manager manually. In that case, you could use something like this as base class:

public abstract class JpaBaseRolledBackTestCase {
    protected static EntityManagerFactory emf;

    protected EntityManager em;

    @BeforeClass
    public static void createEntityManagerFactory() {
        emf = Persistence.createEntityManagerFactory("PetstorePu");
    }

    @AfterClass
    public static void closeEntityManagerFactory() {
        emf.close();
    }

    @Before
    public void beginTransaction() {
        em = emf.createEntityManager();
        em.getTransaction().begin();
    }

    @After
    public void rollbackTransaction() {   
        if (em.getTransaction().isActive()) {
            em.getTransaction().rollback();
        }

        if (em.isOpen()) {
            em.close();
        }
    }
}

It depends on what you want to test. When you have complex business logic in your Finder class you may want to mock the EntityManager - using a mocking framework like EasyMock or Mockito - in order to unit test that logic.

Now since that's not the case I suspect you want to test the persistency of the Employee entity (this is often referred to as integration testing). This requires the use of a database. To make testing easy and keep your tests portable you can use an in-memory database like HSQLDB for this purpose. In order to start HSQLDB, create a persistence context and inject this context into your Finder class it's advisable to use an IoC framework like Spring.

There are tons of tutorials on the internet that explain how to use JPA/Spring/HSQLDB. Take a look at this example project: Integration testing with Maven 2, Spring 2.5, JPA, Hibernate, and HSQLDB