Plugin uses compile classpath in readonly classpathElements

I attempted to add a FIT fixture to the src/test/resources, and a fixture class under test to src/test/java. However the test failed because the FitRunner did not include test-classes in its classpath.

The site documentation mentions the classpathElements variable, but this variable is read-only.

The attached patch removes this readonly limitation, and sets the default to project.testClasspathElements which I consider more relevant for a test-centric Mojo.