Documented why static nested test cases in the spring-test module are
ignored, explaining that such "TestCase classes are run manually by the
enclosing test class". Prior to the migration to Gradle (i.e., with
Spring Build), these tests would not have been picked up by the test
suite since they end with a "TestCase" suffix instead of "Test" or
"Tests".
Re-enabled HibernateMultiEntityManagerFactoryIntegrationTests.
For the remaining tests that were disabled as a result of the migration
to Gradle, comments have been added to the @Ignore declarations.
Issue: SPR-8116, SPR-9398
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