[SPARK-11442] Reduce numSlices for local metrics test of SparkListenerSuite

In the thread, http://search-hadoop.com/m/q3RTtcQiFSlTxeP/test+failed+due+to+OOME&subj=test+failed+due+to+OOME, it was discussed that memory consumption for SparkListenerSuite should be brought down.

This is an attempt in that direction by reducing numSlices for local metrics test.

Author: tedyu <yuzhihong@gmail.com>

Closes #9384 from tedyu/master.