improve JUnit test infrastructure

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

[x] Automatic logout for all component tests in doTearDown()
[x] Support for cleaning side effects such as resetting DB (see TestCleaner from PR #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 #266 - maybe create issue for MTS

oasp / oasp4j