Update TestElementsAnnotatedWith.java to use @compile/proc

In the course of developing regression tests for 6498938 "Faulty comparison of TypeMirror objects in RoundEnvironment.getElementsAnnotatedWith implementation," it became clear that being able to operate on class file for annotation processing tests would be helpful. Now that jtreg has that capability via @compile/process (6799634), the test in question, TestElementsAnnotatedWith.java, should be updated to use @compile/proc.