Add tip clarifying that @Transactional is not applicable to @BeforeClass methods

[asaikali]

This tip is needed because when using @BeforeClass with TestNG tests unlike JUnit by the time @BeforeClass is called the whole application context is setup and and dependency injection is working therefore as a user it easy to assume that @transactional should be working but it dose not work and then you waste lots of time trying to figure out why, until you make a call to TransactionSynchronizationManager.isActualTransactionActive() and discover that @Transactional is not supposed to be used with @BeforeClass when working with TestNG

For example

@ContextConfiguration(locations = { "classpath:test.xml" })  
@Transactional
@TransactionConfiguration(defaultRollback = true)
@ActiveProfiles({ "development", "standalone", "test-db" })
public class TransactionalTest extends AbstractTransactionalTestNGSpringContextTests
{
    @BeforeClass
    @Transactional
     public void setupDB()
     {
          // the following assert fails before there is no TX
          assertThat(TransactionSynchronizationManager.isActualTransactionActive()).isTrue(); 
          // do some stuff that needs a tx
     }
}

[sbrannen]

Hi @asaikali,

I realize this PR has been opened quite some time, so I apologize for that.

However, would you be willing to apply your change to the relocated testing chapter document in master?

If so, I'd be happy to merge that in.

Cheers

[sbrannen]

Related to #16024