[MINOR] Defeat early garbage collection of test suite variable

The JVM is free to collect references to variables that no longer participate in a computation.  This simple patch adds an operation to the variable 'rdd' to ensure it is not collected early in the test suite's explicit calls to GC.

ref: http://bugs.java.com/view_bug.do?bug_id=6721588

Author: Tim Ellison <t.p.ellison@gmail.com>

Closes #6010 from tellison/master and squashes the following commits:

77d1c8f [Tim Ellison] Defeat early garbage collection of test suite variable by aggressive JVMs