Abstract a AbstractPipelineTestCase out of TestDefaultPipeline so common logic can also be used by other test pipelines

I want to add unit tests for page caching valve, and it reuses lots of the TestDefaultPipeline

I also noticed that currently the innerclasses for NewsOverview, Header and DocumentTitle are not actually used during execution of the TestDefaultPipeline :

Also code like below:

HstComponent component = new NewsOverview();
((HstComponentFactoryImpl) componentFactory).componentRegistry.registerComponent(this.requestContainerConfig, "pages/newsoverview", component);

perhaps worked in the past, but certainly not any more, as one and the same component can be inherited at many different locations, and hence the 'id' "pages/newsoverview" cannot be assumed at all.

I also noticed removing the NewsOverview, Header and DocumentTitle does not influence the unit test in any way