[SPARK-7719] Re-add UnsafeShuffleWriterSuite test that was removed for Java 6 compat

This patch re-adds a test which was removed in 9ebb44f8 due to a Java 6 compatibility issue.  We now use Guava's `Iterators.emptyIterator()` in place of `Collections.emptyIterator()`, which isn't present in all Java 6 versions.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6298 from JoshRosen/SPARK-7719-fix-java-6-test-code and squashes the following commits:

5c9bd85 [Josh Rosen] Re-add UnsafeShuffleWriterSuite.emptyIterator() test which was removed due to Java 6 issue