Add echo "Run streaming tests ..."

Author: Ken Takagiwa <ugw.gi.world@gmail.com>

Closes #2778 from giwa/patch-2 and squashes the following commits:

a59f9a1 [Ken Takagiwa] Add echo "Run streaming tests ..."

python/run-tests

@@ -82,6 +82,7 @@ function run_mllib_tests() {
 }
 
 function run_streaming_tests() {
+    echo "Run streaming tests ..."
     run_test "pyspark/streaming/util.py"
     run_test "pyspark/streaming/tests.py"
 }