[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

core/src/test/scala/org/apache/spark/ContextCleanerSuite.scala

@@ -183,6 +183,7 @@ class ContextCleanerSuite extends ContextCleanerSuiteBase {
     }
 
     // Test that GC causes shuffle cleanup after dereferencing the RDD
+    rdd.count()  // Defeat any early collection of rdd variable by the JVM
     val postGCTester = new CleanerTester(sc, shuffleIds = Seq(0))
     rdd = null  // Make RDD out of scope, so that corresponding shuffle goes out of scope
     runGC()