Replace maven-verifier with maven-plugin-testing-harness

The test suite of the maven-sources-plugin is currently based on the maven-verifier. This has the undesirable consequence, that the test uses the installed version of the maven-sources-plugin, as opposed to the local version.

The attached patch replaces the maven-verifier with the maven-plugin-testing-harness, which doesn't suffer from the same problem. Additionally, the speed of the test suite is drastically improved.

For reviewers: When considering the patches quality, keep in mind that the actual Mojo classes and the concrete test classes are almost unchanged. (For the latter, there is the required addition of a protected method getGoal().)

For reviewers: Note the following comment in AbstractSourcePluginTestCase:

• I don't know, why revision 518116 of this class had the parameters
• "properties" and "expectNoErrors". The following checks demonstrate,
• that the parameters aren't actually used and may safely be removed.
• This should be done by the patch reviewer.

I recommend that the reviewer follows my suggestion by applying a refactoring method after applying my patch.