Remove AbstractJpaTests and revise spring-orm test suite accordingly

Status Quo

Except for Spring's legacy JUnit 3.8 based testing framework which is still used internally within the spring-orm module, the Spring test suite was completely migrated to the JUnit 4 programming model in conjunction with SPR-13514.

The reason the legacy testing framework is still based on JUnit 3.8 is that AbstractJpaTests uses a ShadowingClassLoader to load and execute the test in a different class loader.

Unfortunately, it is impossible to achieve the same results without introducing a custom JUnit Runner.

Possible Solutions

1. Introduce a custom Runner that reproduces the behavior in AbstractJpaTests.
2. Migrate the concrete subclasses of AbstractJpaTests to JUnit 4 without the use of a ShadowingClassLoader.

Deliverables

1. Remove AbstractJpaTests along with the remainder of the legacy JUnit 3.8 based testing framework in the spring-orm module.
2. Revise all concrete subclasses of AbstractJpaTests accordingly.