Introduce programmatic tx mgmt in the TCF

Historically, Spring's JUnit 3.8 TestCase class hierarchy supported
programmatic transaction management of "test-managed transactions" via
the protected endTransaction() and startNewTransaction() methods in
AbstractTransactionalSpringContextTests.

The Spring TestContext Framework (TCF) was introduced in Spring 2.5 to
supersede the legacy JUnit 3.8 support classes; however, prior to this
commit the TCF has not provided support for programmatically starting
or stopping the test-managed transaction.

This commit introduces a TestTransaction class in the TCF that provides
static utility methods for programmatically interacting with
test-managed transactions. Specifically, the following features are
supported by TestTransaction and its collaborators.

 - End the current test-managed transaction.

 - Start a new test-managed transaction, using the default rollback
   semantics configured via @TransactionConfiguration and @Rollback.

 - Flag the current test-managed transaction to be committed.

 - Flag the current test-managed transaction to be rolled back.

Implementation Details:

 - TransactionContext is now a top-level, package private class.

 - The existing test transaction management logic has been extracted
   from TransactionalTestExecutionListener into TransactionContext.

 - The current TransactionContext is stored in a
   NamedInheritableThreadLocal that is managed by
   TransactionContextHolder.

 - TestTransaction defines the end-user API, interacting with the
   TransactionContextHolder behind the scenes.

 - TransactionalTestExecutionListener now delegates to
   TransactionContext completely for starting and ending transactions.

Issue: SPR-5079
Merge from sbrannen/SPR-5079

* SPR-5079:
  Introduce programmatic tx mgmt in the TCF