[SPARK-12862][SPARKR] Jenkins does not run R tests

Slight correction: I'm leaving sparkR as-is (ie. R file not supported) and fixed only run-tests.sh as shivaram described.

I also assume we are going to cover all doc changes in https://issues.apache.org/jira/browse/SPARK-12846 instead of here.

rxin shivaram zjffdu

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #10792 from felixcheung/sparkRcmd.