Test case classification and use of test containers for integration tests

[StefanSchubert]

Test needs to be grouped or classified in IntegrationTests, UnitTests and DeveloperTests.
See https://www.baeldung.com/junit-filtering-tests

Acceptance criteria:

• Tests are grouped with the @tag annotation, distinguishing "Integration, Unit- and DeveloperTests".
• Tests are also grouped for specific components and there are TestSuites (again)
  Example:
  @SelectPackages("com.baeldung.tags") @IncludeTags("UnitTest") public class EmployeeDAOUnitTestSuite { }
• Projects Wiki contains a section on the test concept. Explaining the purpose of each category.
• Integration Tests are executed during the CI pipeline build using test-containers
• H2 Dependencies are no longer required within the project, as all Integration tests are based upon testcontainer now.

[StefanSchubert]

Done, merged into main.