Add unit test for the 3rd package-info.java example.

Currently there is not a unit test that matches the 3rd example in the package-info.java file:

``` java
assert_().about(javaSource())
.that(JavaFileObjects.forResource("HelloWorld.java"))
.processedWith(new MyAnnotationProcessor())
.compilesWithoutError()
.and().generatesSources(JavaFileObjects.forResource("GeneratedHelloWorld.java"));
```

As I (and [others](http://stackoverflow.com/questions/32394726/assertionerror-when-unit-testing-an-annotation-processor-with-compile-testing?noredirect=1#comment57279005_32394726)) have had difficulty getting this to work a unit test for this example would be helpful.