make test work on Java 6

Originally used a real NoStackException based on this ctor:

   Throwable(String message, Throwable cause,
             boolean enableSuppression, boolean writableStackTrace)

is a Java 7 only constructor. This caused the build to fail. Switching it to be
Java 6 compliant by using Mockito to mock a Throwable class and return null for
the getStackTrace() method.