Fix defect where a class that is abstract but has @Test annotations w…

Created by: ray-milkey

…as attempted to be instantiated and threw java.lang.InstantiationException

If you have an abstract Java class that defines methods with @Test annotations, buck attempts to instantiate an object with the class type and fails with a java.lang.InstantiationException.

This patch skips abstract classes when selecting tests to run. We hit this error trying to port ONOS from maven to buck.