TransactionalTestExecutionListener currently requires that the
PlatformTransactionManager bean be named "transactionManager" by
default. Otherwise, the bean name can only be overridden via the
transactionManager attribute of @TransactionConfiguration or the value
attribute of @Transactional.

However, if there is only a single PlatformTransactionManager in the
test's ApplicationContext, then the requirement to specify the exact
name of that bean (or to name it exactly "transactionManager") is often
superfluous.

This commit addresses this issue by refactoring the
TransactionalTestExecutionListener so that it is comparable to the
algorithm for determining the transaction manager used in
TransactionAspectSupport for "production" code. Specifically, the TTEL
now uses the following algorithm to retrieve the transaction manager.

 - look up by type and qualifier from @Transactional
 - else, look up by type and explicit name from
   @TransactionConfiguration
 - else, look up single bean by type
 - else, look up by type and default name from @TransactionConfiguration

Issue: SPR-9645