Prior to this commit, the transaction manager and data source look-up
algorithms in the Spring TestContext Framework were not capable of
retrieving 'primary' beans of those types, even though 'primary' beans
are supported in production as well as for injecting dependencies into
test instances. Specifically, if there was more than one transaction
manager or data source bean and one of them was flagged as 'primary',
the retrieveTransactionManager() and retrieveDataSource() methods in
TestContextTransactionUtils would simply return null for such beans.

This commit updates TestContextTransactionUtils by adding support for
looking up primary transaction managers and data sources.

Issue: SPR-13891