This also removes the powermock dependency. Although powermock is a
fine library, it currently prevents dropping Junit4. And since we only
use the Whitebox API of powermock, this simply replaced powermock's
Whitebox with our own.
and FileTestUtil in favor of commons-io. This is required because
Eclipse won't put src/test code into the classpath of src/main
code (even though gradle was configured with an according
dependency).