Support Propagation.NEVER for disabling test-managed transactions

[sbrannen]

Status Quo

Propagation.NOT_SUPPORTED can be used to disable test-managed transactions via the @Transactional annotation as documented in the Javadoc and reference manual, but Propagation.NEVER does not have the same effect even though users might assume it would (see spring-projects/spring-boot#23630 (comment)).

Deliverables

• Support Propagation.NEVER for disabling test-managed transactions
• Document in the Javadoc for TransactionalTestExecutionListener
• Document in the reference manual

[sbrannen]

Pushed sbrannen@47b25eb to a feature branch awaiting the release of 5.3 RC2.