improved mojo configuration in tests

Hi Martin,
as these changes are a bit more invasive than my previous contributions, I'm creating this PR so that we can more easily discuss them.

I came across several problems with incorrect mojo configuration (see previous commits) and finally decided to solve these issues by letting the harness configure the mojos with their defaults. After all, we were already using the harness for mojo instantiation, so this seemed a logical step.
As a result, the tests should be more robust against further extension and modification of mojo attributes.

Along the way, I modified the sample pom because on first sight it gave the impression that it contained an outdated reference. By changing the maven coordinates, I made it more obvious that its version is not related to the version of *derby-maven-plugin*. Just cosmetic, but adds more clarity.