Move SparkR test JAR into Spark, include its source code

One of the `NOTE`s in the R CMD check is that we currently include a test JAR file in SparkR which is a binary only artifact. I think we can take two steps to address this

(a) I think we should include the source code for this in say core/src/test/ or something like that. As far as I know the JAR file just needs to have a single method.

(b) We should move the JAR file out of the SparkR test support and into some other location in Spark. The trouble is that its tricky to run the test with CRAN mode then. We could either disable the test for CRAN or download the JAR from an external URL