Small bugfix for TestNG

There was a bug using TestNG. In the testcase creation wizrd MoreUnit chose the wrong superclass (org.junit.TestCase). This is fixed now.

Missing validation

The new property page did not validate if the user configured at least one folder as test folder for the project and threw a nasty NPE. So I have added some validation and show a error message if the test source folder config is not finished yet.

Another bugfix

There was a problem with sourcefolder in a subfolder (e.g. "test/src").
So I've written some tests for this and made them pass.
Hopefully there are no more bugs because of the huge refactoring for source folder mapping support.

Bugfixing

There were some bugs with the new pre-release, so I spent some time in bugfixing.
Last week I got a mail about a TestNG-problem with moreUnit.
So I'm still working on bugfixing.
As the weather is great at the moment my progress is not so good...