improve JUnit test infrastructure

[hohwille]

We need to improve our test infrastructure to make things easier:

• Automatic logout for all component tests in doTearDown()
• Support for cleaning side effects such as resetting DB (see TestCleaner from PR IdRef feature for typesafe and expressive entity references #617)
• Abstract base class ServiceTest extending SubsystemTest and providing support via ServiceClientFactory
• Rework DbTestHelper and TestUtil
• Agreement on design
  • abstract base classes vs. spring-beans with JUnit profile vs. JUnit rules vs...
  • what to provide in oasp4j-test (framework) and what in oasp4j-template-* (pattern)?
• consider input from (meanwhile closed) PR oasp/oasp4j#207: added example for solution 3 (CSV based test with se… #266 - maybe create issue for MTS