Add the test file to com.ibm.wala.cast.java.test.data/src for pull request #12

(This is a continuation of [Pull Request #12](https://github.com/wala/WALA/pull/12))

Previously, I only included the source file in the test_project.zip file (by hand). I just noticed that the zip file is auto-generated from the com.ibm.wala.cast.java.test.data/src folder. Thus, it is important that the source file for  the PrimAndStringOp test case be added to that folder so that _future_ auto-generations will include the test file (or else that tests will fail because it cannot find that file).

Apologies for the mistake.